My theory? Well, I think the generation I fall into who dug the adventure games during the genre's prime are out of their parents' house and have jobs. I don't have much time at all to play on the PC like I did back in the day.
But then we could beat the dead horse and talk about how adventure games don't sell and that's why you don't see them too often. This results in a loss of general interest because people are forgetting about them.
And then there's us.