They're not tough enough to survive in melee, is my guess. Fenders and corrs both have smaller HP pools than scrappers or stalkers, let alone tanks and brutes. Further with like three exceptions (FF, sonic, electric) none of the support sets have mez protection.
My understanding is those two factors combined led the original devs to say "yeah, that would probably be an exercise in frustration for the players" and Homecoming's generally evolutionary rather than revolutionary development philosophy has preserved the status quo.
I'm not 100% certain that's what's at play, but it's my understanding.