I dunno, maybe just socially awkward?
Once or twice you can say the person wasn't a good friend or the crowd isn't right for you..but if it happens frequently enough and often enough, one should be strong enough to look at themselves at what they're doing wrong instead of going through life playing the victim card.....
Social awkwardness doesn’t mean that you can’t make friends. There are entire social networks based on a group of socially awkward friends who prefer to play World of Warcraft all day and attend sci-fi conventions together. You don’t need to be the life of the party to make friends, in fact most people make friends one at a time, where social conventions play less of a role especially when the other person is as awkward.
Even on that show “Big Bang Theory” you have 4 super smart guys who have a small network of friends.
Any other reasons?