I suppose that even a rather limited selection of craft beers would be a logistical nightmare, and the inventory probaly wouldn't move that well. Most beer drinkers know what a Heineken is, but only a very small segment of Frontier's total passengers would know what a Lakefront Riverwest is. Nice as it might be, it's probably wouldn't come close to covering added cost.
One good thing about just carrying mainline beer selections is that I won't be sitting next to the guy who asks the F/A "What beers do you have -- maybe something more nutty but not too dark. Oh wait, I think I had a wheat beer once, but I don't think that was dark, was it? I think it was Anchor Shore. I want that."