Cause/Effect. It basically got code overhaul with Windows 2000 and jumped from 16bit, to 32bit to 64bit. Don't forget it needs to maintain backwards compatibility. They went with Windows 10 to avoid universal issues with lazy application developers using rehash code from yesteryear and screwing up calls to new commands. Blame the application developers. Windows has to maintain cohesion and compatibility with applications in personal/enterprise realms so they cannot "ditch" the code and start over. They tried with RT and multiple mobile versions before killing them all.
Windows 10 being universal with desktop applets/widgets working across all platforms is new as are the development tools. Cortana is new to desktop. Streaming games from XBox One to Windows 10 computer is new and could change gaming.
The browser issue is being fixed with Spartan (In latest build) which moves forward. Internet Explorer will be phased out as they recognized it's broken beyond repair. They saw Chrome's acceptance and responded. Again, compatibility with legacy applications especially Intranets/Sharepoint sites built around IE's annoying coding will dictate a timely "death" for IE.
It's free unlike other versions of Windows for vast majority of users, again never
before in history of Microsoft. Now they are at least talking making it open source but how that plays out would take serious thought and reassurance to the Enterprise community where actual revenues are generated.