I thought it's because LOTR was well-known, so a lot of people were interested in a movie-version. But it was already quite a gamble, since they weren't sure it was going to work out (earlier movie adaptions were rejected). Starting with the Hobbit, far less famous and not as much movie potential because of limited story line and character diversity in comparison to LOTR was'n't a logical option. I think the only reason they're capable of turning the Hobbit into a movie now, is because LOTR turned out to be such a success. But yeah, movie rights sounds fairly plausible to me too, so let's just go with that ><
......I'm rambling, sorry crying