The same question could be asked regarding Gardevoir/Mega Gardevoir.
The only reason that they are OU and their Mega Evolved counterparts are BL is due to usage.
Why isn't it used more often?
Medicham is already viable in the OU tier and there are better choices to Mega Evolve; for example Mega Mawile gains the ability Huge Power, Mega Gyarados loses it's 4x weakness to Electric attacks, and Mega Venusaur gains the ability Thick Fat which allows it to more easily withstand two major threats (Fire and Ice). Medicham does gain considerable attack and speed but other than that there isn't much that makes it's Mega Evolution more useable than it's normal form.
Tiers are basses on usage, and not power. This means to say that basicly Mega Medichan isn't so high as it is not use so much. BL is however a higher tier than OU, so this does still make seance.
^ The lists are a bit outdated, but the information on tiers is still legit.
^ This list may not agree with what Smogans was at the time, but Smogan is not affiliated with The Pokemon Company, so all lists, even theirs, are just personal opinion.
^ This source is completely reliable for the question at hand.