Just the text book creators being influenced by their biggest consumer.

"Texas buys 48 million textbooks every year. No other state, except California, wields that sort of market clout."[1]

http://www.nea.org/home/39060.htm

IANAEducationExpert

Textbooks are written by publishers and reviewed by boards appointed by the state. However, this is a costly process and most small states do not bother with it. Due to the size of Texas, it makes sense for them to review and direct the writing of their own textbooks.

Thus, small states will often use textbooks originally published and directed by Texas(or another large state) for their school curriculum. This is why it's such a huge deal when <x huge state> removes <x important topic> from their curriculum.

I think the idea of states having national influence in these matters comes from the fact that if a producer of some sort (textbooks, cars, etc) wants to sell something across the US, they need to follow the strictest standards that populous states have, or else they need to have separate cars for California, separate textbooks for Texas, etc. I guess it can be more expensive to do that, so they end up just meeting the stricter conditions nationwide when that makes more sense.

In some states, school districts choose their textbooks. In some states, the state as a whole decides on their textbook. So the states that do that (like Texas) have a much larger impact on the contents, because it's such a large order. Whereas a school district can't afford to have their own textbooks written and published, so they have to go with a version already out there.