I play Brawl because...well it's fun for me. There's also the thing that it's new and I have to fully disect new games, try to learn everything you can. I try all sorts of characters, and my "main" changes about 3 times a week. That's what I find fun about Brawl, it seems like endless character specific tricks, and I really like that for some reason.
But Melee, I love it because of the skill it takes to play it. I only learned about the AT's and all that awesome stuff only last year, and I'm still trying to get some of the stuff down. I probably never will though. The two games have a very different playstyle, and very different measures of skill. Notice how I never stated one was better then the other, so you can't say I attacked -insertgamehere-. I actually praised both. =D