This is because in the books half of the stories that they are showing on the tv show were separated by an entire book. So imagine how much more quickly the stories would be progressing if half of them were held off till next season. If you are wondering why they are doing it differently than the books, its because everyone was driven mad by the release of a book that didnt cover any of half of the story lines (including but not limited to deaneries). I remember it was driving me mad, i could hardly enjoy the book because i just wanted to speed through it so i could learned what happened to deaneries. I cant imagine what it was like for people who had to wait multiple extra years. Anyway the show really took the right direction with this but a slow start to the season is a natural unavoidable consequence.