California has more students (more than 6 million versus just over 4 million in Texas), but Texas spends just as much money (approximately $42 billion) on its public schools. More important, Texas allocates a dedicated chunk of funds specifically for textbooks. That money can't be used for anything else, and all of it must be spent in the adoption year. Furthermore, Texas has particular power when it comes to high school textbooks, since California adopts statewide only for textbooks from kindergarten though eighth grade, while the Lone Star State's adoption process applies to textbooks from kindergarten through twelfth grade.
There is much more to learn about the textbook editorial process in this article, however.