I'm not really buying into the Gardevoir is better than Diancie thing - in fact I think it's nonsense. In a metagame where speed kills, that difference in speed is HUGE. Plus, Diancie has actual mixed attacking prowess (inb4 people say Psyshock) and a better secondary STAB.
The other thing I think that kinda prevents Gardevoir from rising is that unlike some other Megas, it doesn't really check a ton of things by virtue of its typing alone. Okay it checks Latios, Thundurus and many special attackers based on its SpDef, but does not check basically any physical attacker which outspeeds it (ahem fast Chomp pretending to be Tank gets kills on this thing). "Doesn't anything lose to things that outspeed it bludz?" Not necessarily if it has the bulk or typing to take a hit. Gardevoir is actually frail as shit on the defensive side.
Let's go back to the Mega Diancie comparison. Okay, Diancie's defenses aren't all that great either and its Rock typing doesn't do it favors certain times but others it does. It is an effective check to Talonflame, Mega Charizard X (!), and Tornadus-T based on its secondary typing. The extra speed lets it beat out Keldeo, fast Garchomp, Charizard, every other base 100 mon as well as Manectric before mega evolving (Gardevoir can't stop it from Volt Switching out) and Mega Pinsir. Okay, Diancie isn't a fan of Bullet Punch (neither is Gardevoir) or Aqua Jet, and it doesn't stomach Mach Punch as well but it stands up pretty nicely to Brave Bird and Fake Out. It also takes less damage from neutral priority hits because of better bulk.
There is no denying that Mega Gardevoir's Hyper Voice is significantly more powerful than Mega Diancie's Moonblast. It also has plenty of counterplay for switch-ins which are pretty scarce. But for example Metagross can switch into Mega Gardevoir before mega evolving and force it out, whereas it cannot do so to Mega Diancie without Bullet Punch. This is just one example of how the speed tier makes a big difference.
There's a lot of talk about how Gardevoir doesn't really have switch-ins, but Diancie doesn't exactly have a ton of them either. I mean realistically speaking Diancie has nearly as few switch-ins as Gardevoir if you consider all possible sets for both, while it's certainly a bigger threat to offense. Garde's matchup against slower playstyles IS better, but Diancie is still no cake walk for them, plus we have 2 S rank pokemon who don't take up a mega slot and already obliterate fat stuff. I'm not saying there's no reason to use Gardevoir if you can use Hoopa or Manaphy, but they hurt its viability more than Diancie's. With Manaphy at its peak in effectiveness and Hoopa's complete disregard for almost anything slower, this certainly tips the scale on the Mega slot opportunity cost.
