Is it the community?
Is it the increasing faucet?
Is it the good looks?
Is it the good reviews?
Is it the knowledge that they've survived more than a million dollars in loss?
The reputation is certainly the most crucial reason. No one would love to send his bitcoin to a site with bad user feedback. The active community is a plus, and there are not many bitcoin sites that you can have an active chat around the clock. The other three things are not quite important, at least it is the case to me.