Because, heaven as the modern world sees it, doesn't exist... yet... according to the early Christian writings
Ancient writings make a peculiar read. They can subvert our preconceptions, revealing that what we hold to be sacred modern knowledge, is ancient. It can shock the spirit to discover that the ancients believed in a spherical globe of an earth. It can unnerve the inner form, to discover how close the mind of some writers is to the essence of our so-called Western civilization. Augustine's City of God, is often credited as a founding force in our modern world. Virgil's poetry teaches us values such as duty and family, which the ancient Greeks had no concept of.
The early Christian writings are particularly fascinating in their literalism.
The followers of the All Encompassing Church, as early Christians called themselves (even those who disagreed with each other, and formed their own denominations), the 'Catholic' church, to use the modern phrase, which still holds its place even in the creeds of protestant churches... these followers believed in a physical death and a physical resurrection.
The fact Jesus came back from the dead, not as some disembodied spirit, but as a physical creature with real wounds, is definitely of great importance to these early Christians. Roman Catholics even today believe that it is real, living human blood, and human flesh that they eat during communion, the blood and flesh of Jesus as he hung dying upon a tree, a cross as modern language labels the instrument of the death of God.
The dead are only raised, and judged after the death of the universe in the bible. All is destroyed, and human beings are raised and judged. The good inhabit a physical city, with physical bodies, made anew, perfectly made.
The New Jerusalem, Zion, is created after the destruction of everything, and this is where the early Christians believed they would spend eternity, in physical bodies, accompanying their deity.
Heaven, in the sense of the rule of God, of course existed to Christians. Where goodness was on earth, there was heaven. Heaven as a place inhabited by the risen dead however, was a place yet to exist.
In fact, the Catholic church, for centuries has taught that the soul is not separable from the body. It is the blueprint of our body through eternity.
In recent years, as philosophies have changed, these philosophies have influenced religious beliefs also. The idea of an abstract heaven emerges from poetry and philosophy, the concept of saints walking around on clouds, and a hell where the damned are already punished.
These concepts however are modern concepts.
Early Christianity believed in a heaven to come, and in an earth fading away.
The Council of Vienna, in discussing beliefs that the soul was separate from the body, in 1311-1312 stated:
'Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.' (EWTN | COUNCIL OF VIENNE (1311-1312))
The human mind, is essentially a mix of electricity and the reactions of various chemicals. It can be affected by substances, circumstances, even fatigue. To suggest that the mind and the soul are the same thing would seem highly irregular. The soul, if it is inseparable from the human body, can surely then be the blueprint, the writing in of the plan of God for a person. The Lutherans thought this to be an absolute plan, where we are born good or evil. The Catholics however held that choice must be involved, even if God might know the choice that will be made. To the Catholic, mortal sin damages the soul. If the soul is the blueprint of the person written by God, and sin is disobedience against God, then this makes a deep amount of sense. It links with the idea voiced by Jesus in the Christian gospel, that man does not live by bread alone but by the words of God, and to the idea that opens that famous book in the bible, that through the word God made everything.
It's wrongly, but perhaps not unexpectedly suggested by a reader that I was advocating the advent of soul sleep in my analysis. Such a belief, is not what I was suggesting. An interesting article on the controversy over the idea a few centuries ago, also sets out the Catholic position, as I shall quote it:
'We, with apostolic authority, define the following: According to the general disposition of God, the souls of all the saints […] already before they take up their bodies again and before the general judgment, have been, are and will be with Christ in heaven […] and these souls have seen and see the divine essence with an intuitive vision and even face to face, without the mediation of any creature' (Rorate Caeli | A POPE WHO FELL INTO HERESY, A CHURCH THAT RESISTED: John XXII and the Beatific Vision)
However, Christianity from earliest times has held that the spiritual is not physical. The soul, is spiritual, in the same way that the writing which forms the pages of a book is not a part of the story created by the book, but its governing foundation. Heaven in the sense of the presence of God has, as I was aware before the debate, was widely believed by Christians from the start. However, the sense of a heaven that includes any sort of moving about in a body, must be restricted to a heaven to come, for the soul is spiritual.
Spiritual things, like the rules of the physical universe such as gravity, certainly can interact one with the other, just as mathematical formulae interact in the real world. In this spiritual sense, heaven certainly can be said to exist, even in the early Christian writings, it is the presence of God, into which the spiritual soul is governed, awaiting its body, and the heaven to come.
However the concept of heaven as another dimension, or a present physical place existing just out of view does not correlate with the concept of something being spiritual. Spiritual in the sense carried through centuries relates to that which can neither be touched nor seen. The words which create a novel, the code which runs a computer. The writing of God which creates all things.
The modern sense of heaven however, where an alternative realm exists, or people wonder around the lofty heights cannot be said to exist in analysing the ancient writings. That sort of heaven is only something early Christians expected to come about in the future.
Except in the concept of purgatory, where the soul is said to be readjusted before heaven due to slight deviances from God's plan, after the death of a person, in analysing the beliefs in question, their soul cannot be said to change. It, in that sense, of time being the measurement of before and after in change, cannot be said to interact with the divine writing that is considered the soul. Yes, the soul, if it were said to be exposed to time might interact with it in sequence, but could not be said to be changed by the physical world any longer.
However you define the spiritual heaven early and later Christians believed in, the one pre-dating the establishment of a physical heaven, it does not match the concept of heaven the modern world holds. That concept is a mash up of the ideas of a physical heaven, with a cultural shift that envisages almost another dimension, and elysian fields, a mount Olympus, an Avalon. These ideas are foreign both to the bible, and to the early Christian writings.