I can tell you why he stays on the bestseller list. But you will not like it.
His book is a textbook and therefore is purchased by government funded learning institutions. In other words your tax dollars go to this leftist fool so each year his book can teach a whole new crop of young Americans why they should hate America.
The mind boggles at such but alas it is true.
Having to "subsidize" Zinn is like watching our government dollars flow to NPR and PBS. If they were so darn good, they could make money and survive on their own WITHOUT benefit of our tax dollars. Sesame Street used to be a gold mine for its licensees. I hope Bush's theoretically "leaner" budget has some deep surgical cuts for the National Endowment for the "Arts", NPR, PBS, and other leftist voices.