As I understand it the big things are:
1) Texas is such a big market that their curriculum influences the books used in lots of other states
2) On the board that decides this, there are a bunch of pretty hardcore Christianists, just one vote short of the majority. So on each subject that comes up, they need just one swing vote to side with them.
I'm actually a little optimistic about this. I don't think Texas would let the curriculum go full on crazy (ie young Earth creationism). As conservative as the state is, there are a lot of suburban parents that don't want their kids' education being considered a joke come college application time.
Where was the long-form story about this that came out recently? The NYT? I was all tl;dr at the time and never bookmarked it.