as its been stated before, halo and gears of war are more popular competitively because they are:
1. beginner friendly: melee requires much more from a beginner in order to beat a skilled player then do halo and gears. it requires much tech skill knowledge and prowess, much stage knowledge, and alot of character matchup information. halo you can just grab the rocket launcher if you want to beat the guy whos good.
2. lack of gameplay depth: as much as you who also play halo will not admit, shooters are just what they are called: point and shoot. yes there is strategy involved, there are stage techniques and information about each stage to be learned, but the gameplay boils down to: point and shoot (headshot). melee differs in that you must, say for instance with mew2: d-tilt, wavedash (tech chase), grab, a, d-throw, tech chase (wavedash), d-tilt again, tech chase, and either repeat the process or just go for a nair of fair. this is only a small example with one character. the amount of gameplay depth prohibits some people from being good at it, some people just cant deal with all that at once. alot of (stupid) people just like halo for its extreme simplicity, and there are alot of (stupid) people out there like this (no offense to any halo players here), i know alot of them. gameplay complexity is melee's greatest strength, and yet however, when it comes to the mainstream audience, also its greatest weakness.
3. halo and gears of war have blood, explosions, shooting with giant ****ing guns, grenades, and chainsaws. in gears 2 you can chainsaw somebody from the buthole up (i'll be honest here, its ****ing awesome).
melee has jigglypuff, pikachu, fox, kirby, peach, and other friendly looking little critters.
its a large difference in material and content: melee has kid friendly looking characters, and in gears of war you can chainsaw somebody in half. to an insecure male suburban teen compensating for lack of self image, one seems childish and lame while the other seems badass and cool.
there are other reasons, but those are the ones i thought of right now lol, anyone else?