The one book per season pattern worked well enough through the first two books and the first two seasons.
But what really messed it up was the quality of the 4th and 5th books. When they decided to split book 3, Martin optimistically said that they'd have to do the same with books 4 and 5 because they were just as "dense" as ASOS. He figured that would give him a few more years.
The problem was that to many people (perhaps including D&D) books 4 and 5 were dense with boring plotlines. Sure, some people loved Dorne, and the Ironborne, and the dicking around in the riverlands. But it seems like D&D really didn't, and they condensed those two books into one to keep on the schedule. Had those books been better, they probably would have tried to extend things.
I think they did a fairly good job given the source material for this season, and I'm hoping to see it all move better when the next book comes out.