3 mana + equip cost means Mask isn't going to be drawing cards until turn 4 at EARLIEST, which means Curiosity will have drawn 2-3 cards by then. This alone is enough to play Curiosity over Mask, but the other stuff people already mentioned just hurts mask more.
How often does Fish need mana availiable? FOW, Daze? Once Mask of Memory resolves, it is there until the opponent destroys it. (how many T1 decks play green at the moment?)
You either never played fish or never played type 1. Fish needs mana to activate lavamancers and manlands, keep Voidmage Prodigy active if needed, and eats up land drops by using strips. You have a lot less mana than you think.
There's more decks with green than you think, but this point is mostly irrelevant.