lol well alright. basically, melee competition is more reflex-based, while brawl competition is more mental-based. obviously there is a lot of overlap in both (you can't play melee without thinking haha) but i'd say it's like 40mental-60reflex in melee and 60mental-40 reflex in brawl.
this results mainly from hitstun and edgeguarding...in melee, when you're looking to get a hit in, whats your goal? to combo off it, right? get them offstage and kill them early or combo into a kill. however, when you land a hit in melee, you KNOW you have a followup. it's a lot easier on the brain, for example, for a marth to just think "alright, I landed a uthrow on fox at 40%, so I have to watch what direction they go in and turn around accordingly before utilting so I can utilt again" (for example). compared to in brawl, in that same situation, you have "alright, I landed a uthrow, now I have to watch what direction they go in AND guess what they're going to do next (the fox could shine an attempted tipper uair to push himself out of range, or airdodge past it, or jump again). there is no 'guaranteed', so you have to account for more options.
many people say melee is more difficult because of the tech skill barrier required to become proficient at it. also many people think melee is more rewarding because the game rewards you for overcoming this barrier (seamless movement and comboing). while I think the rewards part is up to interpretation (personally I find landing a good string in brawl much more rewarding, but I can see why people would prefer melee's system), the 'tech skill' barrier in melee only serves to alienate. I don't see a reason why people need to learn to hit L every time they land for example, what strategy does that add to the game? it's just a meaningless 'barrier' that you need to overcome in order to even compete.
i guess what it comes down to is brawl is harder because you have to think more. you'll never become physically exhausted from playing melee, but u can damn sure become mentally exhausted while playing brawl. there are ways around it in brawl ofc...but i consider it cheap to play like that