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Because it has an awful case of 4-moveslot syndrome. It needs:
-Play Rough for main STAB
-Fire Fang/Brick Break to hurt Steels
-Sucker Punch to hit things fast
-Swords Dance so Sucker Punch kills, letting it actually sweep.
-Substitute so M-Mawile isn't Wisp bait and can use Sucker Punch and Swords Dance with less risk.
Even then, it's slow speed and low HP, which invalidates it's decent defenses, bring Mawile down even further. When Landorus-T and Heatran are released, M-Mawile will get fukt even further.
Also, it can't use an item, and disallows use of another Mega, many of which hit harder than it. M-Kangaskhan has a hard-hitting Sucker Punch, as well as workable speed and HP. Mega-Heracross has better defenses and speed, and the lack of raw power is made up for by the high base power moves he has access to (Skill Link Rock Blast, Bullet Seed and Pin Missle, Close Combat). Plenty more megas are better than M-Mawile.
