I'm not buying it! Yes, Texas and California are huges states, but the remaining 48 states buy more books collectively than either one.
Perhaps the publishers are foisting THEIR screwed up agenda on everyone, including Texas.
It's true for grades 1-12. Because the purchases of CA and TX are so large (and guaranteed once the book is approved) the publishers don't bother producing alternate books. TX and CA provide the "base" and all of the other states are required to choose whatever books those states have selected because no alternate texts are produced.
TX and CA choose books in alternate years (about a 4 year cycle, i.e: TX-2001, CA-2003, TX-2005). They may choose different books, and other states can buy whichever books either of those states chooses. The publishers produce several different sample books to take to a publisher's book fair where the book committees look them over and choose the text books for their state. Any book not chosen is scrapped -- it doesn't matter how much some people liked it. If it is not chosen by the powerful book committee of either TX or CA, it is not published. The only difference would be that private schools are free to choose their books from smaller publishers, such as Open Court. Private and parochial schools are not constrained to use the choices of the state book committees.
The advantage that the people of TX have is that there are regular citizens on their book committee. It is not dominated by NEA propagandists.
My information comes from my daughter and son-in-law who are in the textbook publishing business.