Talk:Episodes, films, and shorts/@comment-4985604-20130729220201/@comment-6029775-20130729221030

I don't think they ever said definitively (although I'm not too in the loop), but I'd imagine it was because of Equestria Girls. Its original release date was set to be not long after S3 ended I think, so it's pretty possible they were working on them simultaneously. Also it may have been to facilitate the whole alicorn thing a little.