The reason that WWE has been giving away big matches on television lately rather than saving them for monthly pay-per view events is because Vince McMahon is dead set on television these days. With on-going television negotiations, WWE is trying to land lucrative deals for both RAW & SmackDown programming, thus the reason the company has been giving away big matches on these shows.
Another reason for these matches not being saved has to do with the business evolving. Pay-per views mean a lot less now and the WWE Network sees very little movement. Even though WrestleMania and the other big pay-per-views will forever remain a staple for WWE, the focus right now is to make money from the television shows.