The fact that some classes can flat out one shot/bleed you to death/and make you have no chance to attack from start to finish during a match, so there is no way to even consider anything new to this game before balancing out everything within it. Madrid is pretty probably saying a new race will make it even more unbalanced. Elsword is a perfect example of adding new classes without caring about balancing the previous, just becomes more and more broken.
Agree and I would like to add one more example!
Most of the players around here probably heard or even played the game Yu-Gi-Oh... Whether it is the real life card game or online card games, it is still an example. In Yu-Gi-Oh, every now and then, the developers add a new combination of cards that can work together (AKA. Deck).
Decks in Yu-Gi-Oh right now can easily kill a player in one turn... wipe out his 8000 Life Points in one hit, or a series of hits dealt in one battle phase. Yet the developers don't even care, they are just adding more and more card to make the game even weirder (Which was the biggest reason I quit the game). After the developers see how messed up and horrible their creations were, they can't just say they are taking down those cards and waste all the 100s of thousands of dollars players spent on buying them. So, their resolution is to basically either limit the card uses by altering the number of the copies allowed, change some of the text in the card to make it less broken, or basically ban the card.
The suggestion of new races is just like that.