This is so obvious it hurts.
You're too good for them perhaps.
intimidation, risk of being rejected...
You can be too good looking. I dont have that problem but sometimes a person looks so good that peoplenare intimidated or figure you are already with somone.
A lot of people are intimidated by good looking people?
You are unavailable to them
When we sit alone, some people assume we're "trouble", and unworthy of their interest. Of course, jerks like that aren't worth our time anyway, so it's nothing to worry about. :)
If you're not willing to approach people, you can't expect to be approached.