Games are fleeting and will be replaced by other games with different rules THE FOLLOWING YEAR. Major league sports are standardized have standardized rule sets that DO NOT change over time. There are also more overall variables in sports. The human element for one, the way each athlete moves and uses their bodies is inherently different. Thus opening the door for wider variety.
In COD there are class load outs that are better than others due to inherent quarks in the code. I don't follow MLG COD but from what I've seen the load outs and stratifies are pretty cookie cutter. Everyone uses the same strategies and guns. Not that fun to watch. There is also no mainstream audience for it outside of the hardcore MLG community. It won't last forever.