Mawile is still in OU simply because it takes three months for Pokémon to change tiers. The tiers are based on three month usage stats, and because Mega Mawile has not yet been banned for three months, Mawile has also yet to change tier.
>For those new to our system, how our tiers work is that they're primarily based on usage (the names of the tiers, OU, UU, RU and NU, literally mean "Overused," "Underused," "Rarely Used" and "Never Used"): Pokemon that are used heavily in the tier above get banned from the tiers below.
>The cutoff for each tier is the point at which you're more than 50% likely to encounter that Pokemon at least once in a given day of playing, if you play 20 battles a day. For those that don't feel like doing the math themselves, that number is approximately 3.41%. So that means if a Pokemon is used on 4% of OU teams, it's considered OU and is banned from UU, RU and NU.
>This 3.41% number is based on stats weighted over the previous three months, with 5/6 of the weighting from the last month, 1/8 from the month before that, and 1/24 from the month before that (ratio of 20-3-1).