Probably because people feel it unnecessary to talk to a stranger through a secondary way communication. Then again we are all technically strangers to one another even if we are friends in the gaming community To Skype with someone you need a degree of trust and strangers don't have a trust until they get to know them a bit better. Also the fact that when you Skype with someone their Skype ID is automatically put into your address book and some people don't like that.