Spoilers are coming ...
Now, one of the "GoT" showrunners finally explained the reason why.
In an interview with Entertainment Weekly, David Benioff called the information leak "unfortunate," but he revealed that, though there are differences between the show and the books, he and fellow showrunner Dan Weiss are still trying to stick to author George R. R. Martin's various storylines:
Last year we caught up to the end of Bran’s storyline (in Martin’s most recent A Song of Ice and Fire novel, A Dance with Dragon). So if we pushed him forward this season, then he’s way ahead of where the other characters are.
The Season 4 finale of "GoT" leaves off at the point where Bran begins training on his skinchanging and greensight.
Benioff went on to say that it "made sense" to leave Bran's story where it was for now and that going through his training wouldn't be "particularly cinematic."
Though the info from Benioff explains why we won't see much of Bran very soon, don't expect many more details for other Season 5 leaks. When HuffPost Entertainment asked the author of series about released footage that appears to show Jaime Lannister in Dorne (potentially a huge plot change), Martin simply said, "You'll have to discover that when it comes out here. Don't want to reveal any spoilers."