But what to do about them? If the book (picking on these for example) Incarnum and Tome of Magic, did not sell well in the first place, then why would WOTC think a supplement to a poorly performing supplement would sell better. If anything it would sell worse, much worse.
I have no clue as to how well those books sold bs expectations.
However, a little listening to feedback- Lord knows they had a visible presence on various online communities- would have revealed how people felt about the classes. And a "Class Compendium" with revisions (if needed) plus extra material could have been a big seller.
Beyond that, though, even popular classes were orphaned. How many classes got mentions in other, subsequently released products along the lines of "You could use these res with N class, but we advise against it" ?