Now that they already have the base to build the content, why aren't they using it to make a longer game in a shorter time ? Most of the features are already in place (Object Oriented anyone), so all they actually need to do is create the story, design the gameplay, create a bit of new art, make the plot, etc.
From what I've read, their goal is to extend the engine a bit as they go along, like they've already done to add HDR in various places. Thus the Source engine of
Episode 10: Gabe Needs a New Ferrari will be snazzier than the original HL2 incarnation of source.
Plus they can't spend two years developing new content using the plain old HL2 engine, or it will look bad when they release it. They have to keep upgrading the engine, one way or the other.