I couldn't tell you why its taken so long but I could try to explain where the high anticipation comes from and why it is, and isn't highly anticipated.
Half life, upon release was a very different FPS to those currently available at the time. It featured incredible ai (the soldiers especially) and a tight story based structure that had never been pulled off as compellingly as Valve did with their game. Adding to that its sense of mystery and massive modding possibilities it became a dream game for everyone into FPS's.
In 2004 they, arguably, did the same thing when they released HL2. It featured graphics that were beyond cutting edge for 2004 and a story based structure just as compelling, if not more so due to its grander scope, than the first. Plus a further progression of mystery in the story and another easily moddable game. Factor in the physics system which had never been done anywhere near as fluidly or as brilliantly as it was with HL2 and you have another forward thinking ground breaking game.
In short, both times Valve released a major Half Life game they broke new ground. They did something very, very special and created a unique FPS doing things and going places no-one else had even thought of, let alone been able to convey into such a compelling and bloody fun game. It may not seem like this now when you play HL1 and HL2 but it really was when they each came out.
Thats why there is anticipation for HL3. In a way they are victims of their own success, perhaps this has driven the lengthy development period, in that with HL3 they need to do something brand new again plus do everything else better than everyone else. And we want that. We want a new FPS that lets us do stuff we've never been able to do before, and we expect it of them because they did it both times before.
Unfortunately for them given the 10 year wait since HL2, the generation that felt these things for their previous games are moving on, not just to newer games that get better and better in a 'post half life do it better than everyone else' industry, but in age. They have more responsibilities in their life and gaming has less relevance (some of em anyway) so the generation now sitting there where we were in 2004 don't see it as big a deal because they didn't experience that buzz of playing something so bloody special. Thats why it both is and isn't highly anticipated.
Its a lot to live up to and like many I fear Valve won't be able to make it into a hat trick because after 10 years if they don't come out with something no-one has ever thought of and blow our minds to bits again, then HL3 will very likely be the last Half Life game we ever see.
