Just pure old fashion greed.
They are not happy about their current earnings so they are adding another trick to benefit of the suggestible audience
This is honestly a good move for the business, as proven by TERA before where the publisher gained some good cash for using this system
But it certainly is a horrible move from the player's prospect, locking all the good things behind RNG with little chances of getting what you want is frustrating.
At least in TERA, those costumes are tradable if not used by the player. And it is good source for gold there. But the system here is just super greedy, especially from a game that didn't complete its first month since release.
Anyway, I do not think this system will fail, nor do I think much people are going to quit because of this, businesswise, this is a low-risk move, but I hope NCSOFT will respect their audience and not treat them like credit cards.