Quoted By:
Basically because Mawile beat too many of their favorites, like Heatran.
"Why Mega Mawile was Banned
To put things bluntly, Mega Mawile's huge power (no pun intended), combined with all of her other positive traits, made her a massively centralizing force in the XY OU tier. With her Huge Power-boosted attacks and precise movepool, she had the ability to throw off otherwise reliable checks just by changing one or two moves, which allowed her to make the claim of having no hard counters, a claim very few Pokémon were able to make. One of the most common examples of how Mega Mawile can beat her usual checks is her match-up against Heatran. While Swords Dance variants are usually at a disadvantage against Heatran due to its superior Speed and access to Will-O-Wisp, which gives Heatran the capacity to work around Mega Mawile's Sucker Punch, Mega Mawile can turn this match-up on its head just by using Substitute over either Swords Dance or a coverage move.
In addition to Mega Mawile's ability to throw off her counters, it turned out that her one flaw, a pitiable base 50 Speed stat, was not exactly a flaw at all because she could still beat a few notable targets just by investing in her Speed a bit. Moreover, Mega Mawile could use Sucker Punch to get around the faster Pokémon that threatened her. As such, the only semi-reliable checks to Mega Mawile were those Pokémon that could tank a potentially boosted Sucker Punch and/or Play Rough on the switch and threaten her with a OHKO, a task that very few Pokémon in OU could perform. Even then, using revenge killers that met the aforementioned criterion could fail if the Mega Mawile user opted to run Substitute over Swords Dance, forcing players to run more than one Mega Mawile check on their team."
