There are some reason why Mawile is banned and Medicham not.
First: Power, Mawile hurts more
Second: access to sword dance
Third: amazing Typing (Steel Fairy is one of the best types, if not the best defensive type combo in the game)
Fourth: Amazing Bulk + Intimidate, wich combined with the steel/fairy typing gives you an easy chance to get one, or even two sword dances
Fifth: Sucker Punch, fixing the speed problem, and if the opponent uses a status move, the Mawile player can predict that and go for play rough or any move, or even SD again
Sixth: Mawile's only "Counter" was Heatran, that means, to not lose to Mawile you need to bring Heatran in every team, or foddering some pokemon. And even Heatran wasn't safe from the sub punch set
While medicham is faster and has almost the same amount of power, it doesn't has access to any boosting move besides Bulk Up (is not that good), his defensive typing is not perfect, and even when hits 100 speed, it kinda loses to stuff that is faster than it thanks to its poor bulk (Like Gengar or Talonflame), or Pokemon that can take hits from it easily (Like Bulky Psychic types, Mega Sableye, Ghosts in general (specially Cofagrigus) and Physically def Clef)
http://www.smogon.com/smog/issue44/ou-to-ubers
Here gets explained the reasoning behind ALL the bans in Smogon's metagame, and how may people voted to get them banned, check them out