Garchomp was banned because with Sand Veil, you basically had a 20% chance to dodge any 100% accurate attack, which more than likely means good game, whether directly (Garchomp sweep) or indirectly (weakened counters for another dragon, etc).
Whereas something like Mamoswine or Froslass is not going to do anything that detrimental to the other team with one miss. There is a point where banning everything that causes luck to be involved is too far, and banning Snow Cloak/Snow Warning because of Snow Cloak is that point in my opinion.
edit: Banning Brightpowder solely because it raises Evasion, despite no one using it, no Pokemon becoming broken because of it, etc. was a bad decision. If people want to use Brightpowder and lose 80% of the time, then I'm fine with that.
Whereas something like Mamoswine or Froslass is not going to do anything that detrimental to the other team with one miss. There is a point where banning everything that causes luck to be involved is too far, and banning Snow Cloak/Snow Warning because of Snow Cloak is that point in my opinion.
edit: Banning Brightpowder solely because it raises Evasion, despite no one using it, no Pokemon becoming broken because of it, etc. was a bad decision. If people want to use Brightpowder and lose 80% of the time, then I'm fine with that.