Deception based specs have the best burst and situational usage in both 1v1s and small skirmish encounters in every variable warzone situation.

Madness has the best utility and damage over a very sustained period.

Mad Maul tries to have both but fails at both. It really is as simple as that. And even if ideally, in terms of pure damage, it out dps'd Deception based builds over a short period of time, it still wouldn't actually do that because the setup is too long. It has less utility than full Madness as it lacks Creeping Terror root. Instant Whirlwind is good but Low Slash has just as much utility in an organized 8-man team and has a quarter the cooldown. Again, MM tries to get the best of both worlds but is worst at both.