The chests are random drops, the keys are random super-rare drops, and then to make matters worse, you only have a random chance at acquiring a scrap of a worthwhile reward in the chest. Of all the things in the cash shop, RNG chests are pretty much my pet hate. The festival chests were a close second.
I can't remember ever getting a key drop. I've seen less than a handful come out of map completion rewards, though. They and fine transmutation stones really need a chance boost to be more than cash shop bait.