My alternative theory is twofold, and sits next to, rather than contradicts the very good points others have made.
I think some of it is the "nostalgia isn't as good as it used to be" argument. Many people tend to place on a pedestal the matches, players and incidents that first filled them with a passion for the game. Similarly, they tend to appreciate the quality of the players they played against more than somebody they've watched in retirement. So we have a glut of ex-players who are now media commentators who probably don't appreciate somebody like Shoaib compared to somebody like Curtley Ambrose or Jeff Thomson, for the simple reason that Shoaib has never personally made them wet themselves. As a spectator, I know that I'll never enjoy a one-day innings more than the Bevan's New Year's Eve four off the last ball knock. There have and will certainly be better innings played, probably including some others of Bevans, but as a 15 year old that was so damned cool that its on a pedestal in my own mind.
Second, there's an ever increasing proliferation of technical wizardry such as super slo-mo, hawk-eye, replays from ninety different angles, etc, as well as advents like the stats websites etc that make it much much easier to dissect the game, bury us in analysis, and can take a lot of the joy out of watching good performances. I'm not saying that I wish we didn't have these things, or that the game would be better off without them, but while they help us appreciate good performances, they also highlight and expose flaws/averageness. Those flaws/averageness have always been there, but now they are held up much more prominantly than before. I think they draw more attention to negatives/distractions than they aid in the appreciation of good play.
This bombardment of more and more involved analysis of what's going on in front of us, combined with the ever increasing amounts of cricket (when was the last time you saw a ODI, apart from a world cup, that actually meant something to all involved?) has left us all a bit jaded.
I suppose I'm just explaining why I think its important to appreciate the special in today's cricket. In twenty years time, we'll all be crapping on about how Tendaulker Jr isn't a patch on players from the turn of the century like Ian Bell and Andrew Symonds.