I believe they said it was because of trying to ease up server stress for the endwalker launch. Giving EXP after every individual kill across who knows how many instances at once was probably an easy target to get rid of to ease stress on the duty finder server. I believe they did the same thing for spiritbonding calculations too?
Plus none of that EXP is gone, finishing the instance is the same amount, if not more, than it was before.