Simply because it is illegally located on occupied Palestinian lands slated for a future Palestinian state. The Palestinians will never agree to the existance of Ariel as a "major population center", and there is no reason for them to. Israel built the settlement - let Israel deal with the consequences. On a practical note, perhaps the future of Ariel is not so bleak after all. Just like Israel has Arab towns in it, so may Palestine one day have a Jewish town in its midst, provided, of course, that the inhabitants adhere to and respect Palestinian sovreignty and rule.
U.S. and Russia discuss flight safety in Syria on conference call (Reuters)
from the article: Boycott Ariel college