Because it is superior in speed and overall feel versus the runner up vBulletin.
As well, I don't have the confidence in vBulletin anymore. IB released vB4 and it's horrid. It's hard to afford the hosting power just to run vB4.
vB3 is still good, but feels dated.
Xenforo is the future ... short of this lawsuit thing being a bit of a drag .... the future is bright for Xenforo and dark and gloomy for Internet Brand's version of vBulletin. I've seen what the Internet Brands corporation thinks forum software is .... and I don't like it.
I might go back to vBulletin if Kier designed vB5 ... but I'm thinking that won't happen.
Why did you choose it ?