But why are the adventures selling so well now, when they apparently were not a profitable line before? In other words, what made Wizards turn to them as the primary post-core releases, over other things? I know that they are easier to avoid system bloat with, can be stealth-setting books, all that; but presumably Wizards were able to convince themselves that profit was there to be made from adventures in general.
Because selling craploads of little adventures each year didn't work. Selling two big adventures a year does.
Paizo can get away with it because they inherited all the Dragon/Dungeon magazine subscriptions and applied them to their adventure paths.