The most likely 'real' reason for why the main character is human only is because they made a decision that they couldn't voice 3 different races, they had to limit it to one, and they picked human for obvious reasons. That's just wrong, they could've easily given the same voice to each race, but they didn't consider that people living in a certain area tend to sound like other people in that area, not like whatever you think is common to that race. For example, someone in England will sound different from someone from Mexico, who will sound different from someone from New York, who will sound different from someone from Mississippi, who will sound different from someone from Canada, it doesn't matter what race you are, race is completely irrelevant to that. But they couldn't figure that part out, so they decided it was simply unrealistic to have an elf or dwarf from Lothering sound like a human from Lothering and that it was therefore better not to have an elf or dwarf option for the player. I doubt that they'll come out and say that, since admitting a mistake that dumb would be embarassing, but that is definitely my guess as to the actual reason.
Honestly though, even if they couldn't figure that out, they still should've decided that even if they believed it actually 'was' unrealistic for people from the same town to sound similar to each other, it 'still' would've been better to be a bit unrealistic on that point in exchange for the benefit of not having to lose such an important part of what made the original game so good, the ability to create your own unique player from a variety of races. Certainly there's nothing realistic about humans wearing dwarf armor, or dwarfs wearing armor made for humans, but that makes the game 'more' fun, not less, so you can do it. If something makes the game a lot more fun, I'd tend to think sacrificing a little bit of realisticness wouldn't be a big deal, although again I don't think it even would've been unrealistic anyway.
All that aside, no doubt the fact that the PC is human only won't keep it from being an awesome game, I'm totally looking forward to it, and the fact that the PC is voiced this go round shoud definitely make a great game even greater. I played more often as a human than anything else anyway (when I wasn't human I was mostly a dwarf, I didn't really play that much as an elf, although it was definitely fun too, I just liked the other ones better), so it probably isn't going to make all that much of a difference for me that we have to be a human the second go around anyway. I just believe it would've been even better if they hadn't cut out the elf/dwarf options which were obviously loved by all, including myself.