A Storm of Swords took two seasons because it actually needed them. Feast and Dance might have had similar page counts, but I honestly don't think either book was truly as deep as Storm was.
The main reason why Storm took two seasons is because D & D wanted to have the Red Wedding as the big episode 9 event.
The reason why Feast and Dance can't really make for two seasons is because of their structure: you have lots of characters and locations, but none of them have a story for two seasons - and the show can't split the seasons by location, as GRRM did with the books. Jon, for instance, has a great one-season storyline, but splitting it in two just wouldn't work, there is no logical narrative break. Same with Tyrion, or Cersei (in her case though, you may end the season with Walk of Shame - her conversation with Kevan could be moved to season 6). Some other characters have even less than one season material (Sansa, Arya, Bran), probably because GRRM is trying to stretch their story until they grow up a bit more.
Furthermore, some of the characters' stories are already being moved to season 4. So, Feast and Dance are, in a way, being split into two seasons, but these seem to be season 4 and season 5. Theon's ADWD story will apparently be spread over season 4 and season 5. Same with Dany - season 4 seems to be a combination of the ending of her ASOS plot and a part of her ADWD plot. Bran's story will probably be expanded, and we know he'll meet BR this season, so this means that they'll have to somehow expand his season 5, too. Brienne's story is also at least partly being moved to season. How deep any of them go into their ADWD/AFFC material this season is to be seen. I'm only sure that we will see Theon at Dreadfort and helping Ramsey take Moat Cailin this season, but that the fArya plot is going to happen in season 5.