Tanks are completely oppressive in ARAM

Between rapid exp gain, respawn times, champion damage nerfs, healing relics, having the single strongest starter item, and poro summoner spells, tanks outscale almost every champion while having next to no counterplay. You can literally count the number of tanks in the game, and the team with more tanks wins at the very least 9 out of 10 times. There's essentially no incentive to play these games out when you know the outcome. Carry heavy team comps usually don't even take first tower because tanks hit 6 and wipe them. There's no reward at all for most assassins, damage heavy bruisers, or poke champs because their window of power is so excruciatingly small.