Is it possible that Pathfinder has attracted more of a hardcore player + DM crowd, than 4E D&D?
Over the years I got the impression that the more casual and less hardcore players + DMs, were not really interested in buying many splatbooks, modules, and other supplements (both WotC and 3pp). Even during the 3.5E D&D years, quite a number of casual players I knew did not even buy the 3.5E player's handbook.
These days, some of the more casual 4E players I've come across will just borrow somebody's computer with a DDI character builder, to make a character and print out the character sheet with all the power descriptions, etc ... Some of them have not even purchased the 4E player's handbook yet.