One of the complaints about the first set of films was just how much had to be left out because there were only about 10 hours of screen time.
The Hobbit was a simpler book, both in scope and in execution... so they enhanced details that Tolkien (or his editors) thought we could do without. Like a lot of people, I reacted to the news with some surprise... ("Really? THREE movies???") but I don't think this is the main problem at work here.
The real problem is that Mr. Jackson gave us again exactly what we liked so much the first time around, and we're not impressed this time like we were when it was new. Giant, CGI armies were impressive the first time around. Not so much the second. The incredible, panoramic landscapes of "Middle-Earth" were fabulous the first time around, but now... seen it. Elaborately staged sword-and-archery battle? Yawn.
EVERY series that runs on too long starts to have this problem...and many, if not most, ultimately fail. Sticking with giant, $200 million blockbusters, look at how the Matrix trailed off. Pirates of the Caribbean. Indiana Jones. The Batman films that followed Burton. Sometimes, they falter and gain their footing again... The X-Men films, the Bond films (Moonraker? SERIOUSLY?)