I think part of the reason is because lots of characters' recoveries can be difficult to challenge, with Mewtwo's being one of the best in the game. Teleport is sudden, moves Mewtwo very quickly, ledge-snaps, and Mewtwo is completely invulnerable during the actual teleportation. Plus, Mewtwo has high airspeed and a healthy set of aerials that, when combined with his cancellable midair jump, allow him to challenge most opponents in midair. And, on top of all that, Teleport is almost always a completely optional move, since Mewtwo's insane midair jump allows him to recover from almost anywhere onscreen, and, if that doesn't work, then he also has his wall jump and Confusion to fall back on. Essentially, Mewtwo never has to enter his helpless state unless he wants to, and he has a nigh-ungimpable, highly varied recovery with impressive midair options to boot. Oh, and Mewtwo's ledge-game is pretty nice, too, since he can use Confusion and Disable when he's coming back onstage.
Basically, I think that most opponents think that attempting offstage edge-guarding against Mewtwo will put them at greater disadvantage than if they stick to onstage edge-guarding. Yeah, it might not always work, but at least they're not putting themselves a disadvantage. That's my best guess, at any rate.