My guess has more to do with sales and marketing than anything else. I've got more than a few textbooks from the 50's and 60's on a variety of subjects, and fo

My guess has more to do with sales and marketing than anything else. I've got more than a few textbooks from the 50's and 60's on a variety of subjects, and for the most part, they are a joy to read, because they assume the reader is intelligent enough to not try and eat the chalkboard erasers. Big textbooks, with lots of diagrams, pictures, and colors, look impressive. They give the impression of a tome filled with knowledge and wisdom, whether or not they actually are. Which is why school districts and professors in charge of lower-division courses buy the damn things -- they get impressive-looking textbooks to wow the students and parents, and they don't have to think much about the choice. It's just good marketing.