I'm no filmmaker, but I can't think of a way to get all that backstory in without a lot of exposition (characters talking to one and other), or narrated flashbacks like the one that opened TLotR. Frankly, too much of that would really bloat and slow a film, and in fact Tolkien didn't spend tons of time on it in the main narrative of any of his books. It gets in the way of telling a good story, and properly belongs in appendices or expansions that fans can seek out if they want to know more.
A lot of that was answered in the LotR book series. Not in the same massive degree as his Similarian appendix, which is one boring ass read, but those details were there in the books. I'm the type of person when I have a story of a classic antagonist and protagonist I LIKE knowing the motives of BOTH. LotR as a movie series did nothing to give us motives. No backstory. Nothing. In fact, it's very confusing unless you've read the books or can just competely disengage your brain about the story elements that are completely devoid in the movies and just enjoy the banter/action sequences. All the characters, which admittedly there were a lot, were all 1 dimensional and shallow. The only real characters even remotely developed in the LotR movies were Frodo and Gollum. More Gollum than even Frodo the main protagonist. At least Gollum had a back flash scene for him. Although Gollum is a very simple character with a single driving motive to him which is possession of the ring and nothing else.
This is WHY I am liking the Hobbit in comparison to the LotR movies which I found so-so if only on the basis they were a good "flick" and not a great movie narrative. The Hobbit is actually developing more of the characters. We are getting more about Gandolf, and the wizard circle, more about Bilbo, and more about the dwarves. Of course not ALL the dwarves are getting as developed as the book did, but really we don't need that much development on all them.
Even in the Hobbit, only about 8 of the dwarves had any real character development to leave a lasting mark. Thorin, Bofur, Balin, Kil, Fili, Ori, Dwalin, and Bombur were the ones I mainly remembered. This even after 15 years since last reading the book. I instantly recognized them on the screen too. The rest in the book weren't that fleshed out. And so far the movie seems to be doing about the same. Thorin being fleshed out the most, as he's the dwarven prince. Balin is his main right hand man although a bit seasoned, but the brightest of them and his main counsel. Bofur is the most friendly and jovial. Ori one of the youngest and a bit on the stupid side. Kili and Gili the young mischievous brothers and the youngest. Bombur the biggest dwarf (at least in girth) of them all. Dwalin the hardiest/strongest warrior of them.
Although, as a massive spoiler, the main reason Thorin, and eventually Kili/Gili get a lot of character development attention is because they are the three that die in the big battle royal at the end of the book. All of which were in line for the throne. And since they all die, Gimli, is now the sole heir for the title of King under the Mountain.