Multitude of reasons, some with Steam, some not, but mostly because back in the early days of steam (when Steam Early Access meant that you were beta testing the service itself, not some game), Valve, being the ignorant assholes that they were, couldn't tell the difference between a modder, a hacker/pirate, or an in game cheater and banned everyone all the same, and if you argued and even proved to them they were being overzealous douche bags, rather then reinstating your account or refunding your money, they perma-banned you and kept your money. To this day they give a giant middle finger to their earliest adopters and beta testers who made their service what it has become by refusing to unban those banned on the grounds of things they allowed after releasing to the public.
MPlayer was the best, and not because I was a Sage who routinely abused power for comical reasons with hilarity ensuing, but because when you're drunk at 3am and eating greasy ass tacos from the local 24 hour Taqueria and deciding that a machine gunner in C&C should have a 50 round burst instead of the 5 rounds and doing to a crowd of soldiers what a machine gun would actually do to a crowd of soldiers and finding a match with players whom also were probably drunk and would like to see that exact same thing happen, and no bullshit service telling you that you're banned because everyone playing had the exact same files but they weren't the official ones... To watch mPlayer get absorbed into GameSpy and then see their deformed three way love child with Valve become Steam... It's just very disappointing.
Don't even get me started about their TOS agreement telling you they want and will get full read and write privileges to every file on your hard drive... Because no one has ever exploited a security loop hole like that before... 
