Okay, here are the reasons, so far, that I've seen for Heavy Brawl:
1. It makes combos possible:
Combos are already available in Brawl, if you'd just take the time to try it yourself or just look around the forums for a few minutes. Evidence of comboing is all over the place!
2. It makes combos easier!
Making combos easier removes depth. Removing depth removes elements of competitiveness. Also, combos are only easier because it allows you to use playing styles you picked up in Melee. It's time to grow up and try something new.
3. It'll make certain characters better!
It'll also make a bunch of characters much much worse. The lack of l-canceling will make it a shield-grabbing fest at the expense of heavy and medium characters while the lack of vertical movement will make it nearly impossible for certain characters to come back easily.
4. Okay, but isn't the point of competitiveness supposed to be about making combos all day long?
No, that couldn't be further from the truth. What makes a fighting game competitive is the depth of the game, which almost always winds down to the manipulation of advanced techniques, understanding of a characters, and ability to play effective mindgames.
Look at games like MvC2. You can do combos out of the wazoo, but the game lacks such little depth because characters have little to work with. Compare that to Third Strike, where characters that can combo like crazy, like Ibuki, fare rather poorly whereas hard-striking characters like Ryu and Urien are top-tier because of their ability to confuse and manipulate the opponent with mindgames. Same thing with the DoA series: Jann Lee is a top-tier character even though he's a strike character and not a comboing fiend. Ayane is top-tier and she can combo, but her greatest strength actually lies in her ability to psych out the opponent with Tiger knees. Again, look at a game like the SoulCalibur series: Sophitia and Cassandra's strengths lie not in their comboing abilities but their crouch dashes that allow the opportunity to open the opponent to an ***-whooping. It's not all about combos, it's about how you USE those combos.
5. Okay okay, but why don't we just give it a chance?
I'm willing to give it a chance, but not as the competitive standard for Brawl. The problem with doing that would be the inability to determine where you'd draw the line. Oh, so Heavy Brawl is okay? Why not then modify the damage ratios like crazy? Hell, why not make everyone Metal and give them Spicy Curry breath, that allows for awesome combos!
What I'm trying to point out here is that you need to have a standard that's easy to agree upon, and what standard is easier to agree on than the standard Brawl itself? Basically EVERYONE will have played it, so it won't be a matter of being able to manipulate the game in a better way that suits certain people better. At the same time, the game was designed with regular Brawl in mind and a lot of moves seem to be built for it, so it only makes sense to stick to the standard Brawl as the competitive standard than the Heavy Brawl simply because there's a lot more things to be discovered.
But it looks so much cooler!
In my opinion, Brawl looks pretty **** cool right now. It's not as fast as Melee was at its competitive prime, sure, but then again early Melee videos weren't very fast either. Give it some time.