But more than that, there are laws, that were written perhaps in part due to pressures from these groups, but in part from all kinds of other pressures, which provide a lot of inertia for the way things are done.

Whenever there's some kind of education reform, it's usually to add stricter standards, because we're always falling behind in education and feel that stricter standards will help make sure that we don't. This means that textbooks need to meet stricter standards. Which mean that there needs to be some kind of review. Which means that someone needs to pay for that review. Which generally falls on the textbook publishers, or some overworked underpaid government committee. Which means that only a limited number of textbooks go through that review, locking out anything else which might be suitable but hasn't been reviewed to determine if it meets that standard.

The standards wind up changing every few years, so the textbooks need to be updated to match.