Possibly that is because each of the original "X-Men" had only one obvious power "Cyclops'-Energy blast, "Angel"-Wings (flight), "Iceman" - Cold/ice ,"Beast"- Acrobatics (you might add prehensile feet to that)."Marvel Girl"- Telekinesis. They were also first introduced as a single team, unlike many super teams that are collections of heroes with existing back stories (e g JLA, Avengers etc).