I have to say that I also enjoywed his performance. Hollywood loves a comeback saga and I am sure they would love to give the oscar to Rourke for this reason.
One major reason for the Wrestler not beng nominated might be fro the following facts:
1: It was filmed more like a documentary than a film. If you look at "Beyond the mat" you should see several similarites, especially the bits with jake ther Snake Roberts. The only main difference between "BTM" and the wrestler is that rourke never addresses the camera.
2: It was filmed in only 35 days. If youlook at previous Oscar winners it appears that the Academy likes to reward gfilms that take longer to make and have an epic field to them.
3: It also seems that it was an simple film to make, there were real difficult setups, perhaps the biggest issues might have been setting up the crowds for the biger venue. If you compaer this to "Dark Knight", where it is alot of work had to be done setting up the shot before filming could even begin. In some cases it was several months.
4: My wife and my sister detested it. Therefore this means that not everyone got the film.
On a side note: Whether it got nominated or not does it really matter. Do we really care about the oscars any more. As someone has pointed out, the only really important things at the oscars is what the women are wearing.