FFA lets you get away with a lot of stuff that will never work against good players in 1vs1. Even in the rare occasion your typical FFA player meets a great player in FFA, he can simply avoid him alltogether and go chase the lesser players. Sure he might not win but he never was forced to deal with said good player.
So FFA really gives zero incentive for said player to get better. Thus never learning from their mistakes.
Most players would have to completely change the way they play and view the game for 1vs1 as so much of what they do is fundamentally bad. And your typical player, especially that bought this game, will unlikely be able to do this.
As a result this board would be flooded everyday with:
"zomg cheap players spamming 1vs1 sux" "zomg I keep using X move and it doesn't work wtf hax 1vs1 sux" "zomg no room to breathe players won't let me escape how cheap 1vs1 sux" "zomg X punishable move I can't deal with is OP and broken nerf PLZ 1vs1 sux!!!"