Nah, almost never. That is usually forced by the university, publishers and, surprisingly, by the students and their parents:

Say the textbook used for the last 5 years costs $300 and now you want to switch to a different, cheaper book. Now you have to convince admin (why switch? The previous one works) and students (I pay $$$ tuition, why are you cheaping out on the book? Is it worse?).

> how much royalty do they get?

Basically nothing, so that is obviously not the reason and you should be ashamed of yourself for suggesting that.