I think they're possibly waiting for technology, each HL has pushed the boundaries of current technology in a massive way, so I suspect that they're waiting for a gap to open up to create something big, and I suspect that rather than it be episode 3, it will be HL3.
Then again, games companies do confuse me with their decisions sometimes, for example, Square-Enix would solve most of their financial problems if, rather than releasing new Final Fantasys which are mediocre at best, they went back and updated the graphics on the ones from seven onwards.
Heck, just releasing seven with better graphics would probably keep S-E in the black for a decade.
