Why isn't Fur Coat banned if Ice Scales is?
Basically Frosmoth is also a decent evolution, while Alolan Persian is not. Frosmoth gets access to several good moves and has a good stat boost on top of that. Persion just provides 25 all around, while Frosmoth provides 40/40/25/80/60/45 stats. That’s also after providing Quiver Dance.
The second quickban is something that was agreed upon very quickly, and seems to have a fairly high amount of agreement among the community as well; Ice Scales turns any Frosmoth inheritor into a stupidly bulky special wall that can easily set up on nearly any Special attacker in the metagame as well as passive Pokemon such as Wailmer or Tangela. It isn't a slouch defensively, either - 40/35/60 defenses with Ice Scales turns practically any Pokemon with reliable recovery into a nearly unbreakable Special wall with Special bulk rivaling that of fully invested Eviolite Chansey. With its access to U-Turn, Dual Screens, and Defog, it can almost always switch in on any Special Attack while still providing team support. Although it certainly struggles with Physical attacks, the extreme splashability of any of its sets goes a step beyond invalidating Special attackers - it actively punishes even using them. Due to the aforementioned invalidation and punishing of Special Attackers, its sole abuser's far-above-average stat bonuses, as well as a consensus within the community, the Cross Evolution council has decided to quickban Ice Scales as well.
However, the thing pushing it over the edge is halving all special damage, which both can be used to make unkillable special walls or guarantee set up with Quiver Dance or Dragon Dance. Frosmoth is a fine evolution with Shield Dust, but not with Ice Scales, while Persian asks you to use a pretty crappy evolution just for Fur Coat.