Why? Some thoughts: As an English teacher, I don't teach many lessons. So my classroom blogging consists of a lot of "here's what we've been doing" as opposed to "here's this lesson to teach this concept, how could I improve it?" I do teach some lessons, but not as many as it seems Math teachers do.
I also think that unfortunately Twitter sucks up a lot of the mentally energy and brain time of teachers that are online. That leads to a lot of consumption of what others are doing. Obviously someone is creating what's being consumed but there's not the balance there could or should be.