It might sound like a fighting game-elitism thing or something, but I can't describe it any more than saying it's different. The timing on pulling off a huge combo in a fighting game is so far removed from the computationally simple matter of aiming and shooting in an FPS that online play completely loses the complexity. For example, check out this guide to a character in Marvel vs. Capcom 3 and take a peek at some of the videos if you can. Looking at the string of inputs, and realizing that every single one of those is measured in frames and fractions of frames, it should be apparent why dropping a single packet of information could be vital. Zero has some of the more complex combos in MvC3 and, back when I was interested in the game more, he was considered a high-tier character in offline play, but executing his advanced combos was nearly impossible online, even if you could get it perfectly in local play every time. You have to vastly adjust your timing and at best only get part of what makes his combos so powerful. A character with less complex inputs automatically gets raised in tiers, often far more than they ever could have been, while complex and timing-intensive characters frequently get shafted. Competitive fighting game play online simply is not the same as locally, and local competition used to be available in arcades.