Most of my reasons have been mentioned already, but here ya go:
First and foremost, it's fun. That's the basic reason for playing any video game in my book, and Brawl is definitely one of the best in that area.
Second, variety of content. Compared to it's predecessors, Brawl just has so much more to choose from. In the previous two games, I could only find a couple of characters I liked playing as - Marth and Link in Melee, Captain Falcon and Link in 64 - and a couple of stages I really liked. In Brawl, I have three mains I love playing as (Lucas, Dedede, Falco), plenty of others I'm not as good with but still love playing, and like all but 9 of the 41 stages. Not to mention that I like replaying SSE every now and then, a hell of a lot more than I did replaying Melee's adventure mode, and Boss Battle mode is great as well. Plus, I'm an old Sonic fan, so Sonic vs Mario in Brawl is a dream come true for me.
Third, Wifi. Combined with services like SWF's Friend Finder, it's a fast, easy way to find a match with another human whenever I like. Much better than playing computers, which was the only alternative in Melee/64 if you didn't know someone who was willing to play any time. And I have no difficulty adjusting to the minor input lag most of the time, so that doesn't bother me either.
And of course, unlike old Melee tournament players, I never cared enough to bother learning to implement wavedashing/l-canceling into my playing in Melee, and consequently Brawl hardly seems slower than Melee to me at all. And in any event, speed has never been a big reason I liked the smash brothers series anyway, so even if it did, I'd not likely be bothered by it. So that reason for old Melee players to prefer Melee doesn't apply to me.
Zevox