Some issues don't show until you get more numbers on it, QA would have only been looking for actual bugs in the way the code works, but balance issues can only be found by larger scale play (hence the PTS server which is closer to "real" player numbers on a server).
Considering the number of class changes they made, there was always going to be issues with something like this, and putting it off until those are sorted is a sensible move for them.