Because Skype always works. For example, most ISP in Russia still allow access to provider's network ever if internet connection is unpaid. Skype works, because somebody with internet who paid for it is gate for all unpaid users.
Skype traffic is almost impossible to block except some hacks about detecting his autoupdate.
Works usually yes, but that isn't all a good service needs. I'd like to enjoy using it rather than getting Skype-rage. Interface, inability to block calls (from iOS at least), I'd better not start or I won't stop.