1) Having played both I feel like Brawl is more about your ability to read your opponent than Melee is and I enjoy being able to read and outplay people. It's more about one player's ability to out-think and outplay the other than it is about who has faster hands. I wasn't really big into the competitive Melee scene, but I feel that it's harder, mentally, to be good at Brawl than it is in Melee.
2) On that note, because I feel that way it also feels much, much better to win in Brawl than it does in Melee. The reward is greater for me in Brawl, so I like it a bit more
3) This is going to sound ******** on a competitive level: Brawl has a lot of subtle fun elements to it than Melee did. Like at the last PBT tournament when I was playing against Daimonster in the finals and he counterpicked Pictochat on me. At one point the springs at the sides of the stage spawned and I completely dropped the fight and ran for the spring and started doing Metaknight's downB on it over and over and he yelled "I have no idea what to do!" That **** was fun
4) Metaknight. He's extremely fun to play as and against at a high level. (Yes, I just said I enjoy playing against Metaknight) The Metaknight ditto is fast paced, requires extremely good timing in order to be able to punish properly and makes you play at the best of your game in order to be able to win. Plus he's awesome. Did you know his cape turns into wings? </inside joke>
5) The community. The people I play Brawl with week in and week out are ****ing awesome. It's SO much more fun to play competitive Brawl than it is to play, say, competitive Magic. The Brawl community has a lot of the same people in it, but it also has a lot of people that are only in it because this isn't Melee.
6) It's fun. That's it, I just have more fun playing Brawl than Melee. Melee turns into a math homework after a while, where Brawl is still offering new experiences to me every time I play it.