Acid enhancements come from emeralds, which are the rarest and most expensive emeralds available on Layo, you can see that when someone holds an auction to sell one.
For a reason, there are fewer monsters who can resist acidic damage, now, if an item would be created that simply allowed people to use acid at will, i believe it would require additional coding and monster property changes to balance the world. Don't think the team has that much free time, since i guess they have a lot on their plate right now.
I'm not saying this because i own an acid weapon, it's obvious that we players are constantly trying to get the best equipment possible to fulfill their tasks, so if Fire/Frost/Electricity would be the element that monsters resisted less, maybe i'd be using it. It's also up to the player and the RP aspect that can dictate which element you'd use.