Well, to be fair, not many X-men characters, or "solo" heroes at all, have their own comic series. I'm by no means a rabid comic beast, but everytime I'm in a comic store, people are talking about Wolverine. Everytime I walk by a display of the month's popular comics, something featuring Wolverine is on there. I think it's safe to say he's one of Marvel's more popluar heroes.
The same can be said of the movies, really.
Also, most would admit that Wolverine does indeed have one of the more interesting origin and backstory. It's certainly more entertaining than Cyclops'
In fact, I can't think of too many X-men that have interesting backstories, since they tend to join groups, be the offspring of heroes, or be found by Professor X/X-men before things get interesting.
I think it all boils down to the series being 3 movies in, and the possibility of a 4th only being that, a possibility. Logically to squeeze more money out they wanted a prequel, and really Wolverine is the only choice here.
That all being said, I'll be the first in the line here to see it. I love Wolverine. Alberta doesn't have too many heroes