Again, supply and demand?
No matter what you think of the quality, Keep on the Shadowfell probably outsold anything that Paizo or third party publishers have sold...
There's something in that.
WotC, by virtue of the size of the company, require each product to sell X units in order to be worth producing. Paizo, Goodman, Green Ronin and the like, due to having smaller overheads, need only sell Y units, where Y is much, much smaller than X.
Additionally, role-play heavy adventures appeal to a very limited subset of groups: those who will run the adventure (close to) as-written. By contrast, combat-heavy adventures are more flexible - given an interesting locale and opponents a DM can adjust the adventure to be find-the-mcguffin, rescue-the-princess, or whatever style of adventure he wishes. As such, they are inherently more likely to sell well.
That being the case, WotC can't really be blamed for going for the 'low hanging fruit' that dungeoncrawls represent. I suspect even these adventures are barely profitable. (However, with Dungeon back in-house, and on a subscription model, there is an opportunity to take more risks with adventure styles. If two adventures per month are good quality dungeoncrawls, good enough to sustain sales, this would allow them to experiment with the third, if they were so inclined.)
And it's not as if the third-party produce a lot of RP-heavy adventures. As noted, Goodman and Necromancer specialise in old-school dungeoncrawls (although, I've noted a surprising amount of RP-opportunity even in something like "Castle Whiterock"). And Paizo have produced their fair share, with "Age of Worms" and "Seven Swords of Sin" immediately springing to mind as examples.
Besides, good RP-heavy adventures would appear to be extremely hard to write well. Sure, the Freeport trilogy were solid, but the WFRP adventures I've read (also from Green Ronin) are rather poor. (Specifically, the "Paths of the Damned" trilogy, and "Barony of the Damned".) Given the choice, I'll take a good dungeoncrawl over a poor RP-heavy adventure any time.