Habboi, I don't think it's about technology here. I doubt they're waiting for the technology that they'll grant them the power to simulate puzzles at an atomic level.
They are probably waiting for the FPS gaming market to die out. Every new sequel to MW or Battlefield and other examples seem more and more superficial(not that I've played them, but at least if that's not true, then it is true that the gaming market is saturated with this genre).
Then they'll make a big hit with their game. Or who knows, perhaps they're hinting at a new engine they're developing.
Also,
you gotta love Google Trends. Perhaps Valve is waiting for people to start forgetting about HL and come with the sequel as a surprise. Although I could say the trend indicates that interest began dropping not long ago, it's actually kept a "steady"(ignoring the spikes that might indicate other rumors along the years as well) line since 2008.
Notice the last days. I'm waiting for another week until checking the trend again.