Maybe they've gone the way of bethesda and have lost their interest for the half life series. Granted a 3 year hiatus to rekindle their interest in the series is a little much but still it's a possibility.
(Yes I know bethesda will eventually make another tes game. It was the easiest comparison I could come up with.)
Yes, but TES: Oblivion, was a fully completed story after Shivering Islands (I dont think I've seen a more conclusive ending to a story then having your main character actually becoming A GOD!).
It makes sense for them to then go work on something else because that story is over, so they'd have to start something completely different for the next game.
But this is like Bioware decided to wait for 5 years before starting to work on Mass effect 3, and instead pumped out only Dragon age titles and even a new Jade empire game. It's great, but I want to see more of their best series and see the conclusion to the story.