Lot's of shitty books of all kinds sell because of good marketing. It wouldn't be a phenomenon exclusive to textbooks.

That said, there are huge differences among calculus textbooks. I don't know the one mentioned here, but when I studied, there was a choice between a very concise and condensed one, and a very drawn out and long one. The latter would have bored me to death. It is not easy to present maths in a short and concise way.