Many of the 50D's got snapped up because a lot of people think that it was the last of it's kind. The X0D line of Canon cameras was well loved. It started with the 10D, although that wasn't really a good model. The 20D came next, as was pretty much the 'best in class' for the next several years. The 30D was just a 20D with a facelift.
The 40D brought some nice technology upgrades, and at 10 or 12 mega pixels, some think that this was the perfect sweet spot for this type of camera. The 50D was another technology upgrade, but from most accounts, the image quality didn't improve from the 40D (although it does add more pixels, so even at the same image quality, that is still an upgrade). The 50D also added video to this line of cameras.
Then the 60D came out. It was completely redesigned and much was changed. The 60D is much closer related to the Rebel line of cameras. Some even call it the 'Super Rebel'. It seemed like a funny thing for Canon to do, but actually it makes sense to me. The 40D & 50D were getting to be pretty close to what the new 7D is. But the new 60D is pretty much half way between the 7D and the Rebel T3i....so it gives the Canon EOS lineup, a better balance/spread.
Also, the 60D (like the 7D & T3i) are heavy on the mega pixels, especially for an APS-C sensor. Many believe that this hampers image quality and may require high end lenses to really get the best out of the sensor.
But that being said, the 60D is still a nice upgrade from the T3i. Not necessarily in terms of image quality, but certainly in the fit and feel of the body and the controls etc.
So that's why I think the 50D is hard to find.
The D7000 is a nice camera as well.
