My theory is that since HL2 was "revolutionary" because of its physics engine (and HL1 before was "revolutionary" because of the story telling and no cutscenes)

My theory is that since HL2 was "revolutionary" because of its physics engine (and HL1 before was "revolutionary" because of the story telling and no cutscenes), it makes sense that HL3 should combine the series excellent storytelling and level design with something that is also considered new for its time. The problem is that the only "new" thing since HL2 that could take off is VR which isn't quite there yet.