Balance issues, likely, and people trying to rig the system. Especially given that AST will be getting a rebalancing soon, this could be a very hard thing to do without making them more useful than WHM or SCH.

I think the most effective thing to do, personally, would be to add a skill that allows you to pick a card of your own choosing. Perhaps it can have a long cooldown (120-180 seconds) to justify it, but it would certainly make AST more viable*. As it stands now, I hate when I draw Ewer twice in a row (even with a shuffle) when I have a Warrior, Bard, and Ninja in my party.

And then I use Royal Road and draw it a third time.



*This isn't to say that AST isn't viable; I love the class, but it does have a much harder time picking the party up after a mistake than, say, WHM.