Why didn't Spider-man join the roster of X-Men? EDIT-( Until I asked this question, I had always thought he was a mutant.)
Although the X-Men have occasionally taken on non-mutant members, Spider-man hasn't shown interest in joining. Until recently, Spider-man had always been an independent adventurer and did not join up with other super-heroes in teams.
In the current continuity he is both a member of the Avengers and the FF. This is a new role for Peter Parker, who has matured since his youthful independent days. So it's possible, I suppose, that he could someday join an X-Men franchise - but since he's booked up with Avengers and FF that currently seems unlikely.
I hesitate to answer in case this is too obvious but... because he is not a mutant. He has superpowers from a coincidental encounter, he doesn't have the mutant gene specific to those who become X-Men.
Spider-man initially was not thought of as Mutant because it was radioactivity from the spider bite that was thought to have caused him to change into the Spider-man we know. He really did actually mutate but Professor X did not want to go down another path of mutations unless they were actually born with the X-Gene. So Peter was never really born with this gene so technically not a mutant.