While I can't argue those numbers, Gardevoir doesn't really want to fully invest into attack when Double-Edge is her only viable physical move. Sure, she has Giga-Impact, Facade, and Return as well, but those are situational at best, and they would all be Fairy-type anyway. No, she wants speed and whatever bulk she can get, which is something that Double-Edge impacts, and she wants to support her main sweeping moves. When you think of it like that, and then compare fully invested Psyshock to minimally invested Double-Edge, you get:
252+ SpA Mega Gardevoir Psyshock vs. 252 HP / 252+ Def Blissey: 312-367 (43.6 - 51.4%) -- guaranteed 3HKO after Leftovers recovery
0 Atk Pixilate Mega Gardevoir Double-Edge vs. 252 HP / 252+ Def Blissey: 265-313 (37.1 - 43.8%) -- 99.8% chance to 3HKO after Leftovers recovery
252+ SpA Mega Gardevoir Psyshock vs. 252 HP / 252+ Def Eviolite Chansey: 228-268 (32.3 - 38%) -- 96.3% chance to 3HKO
0 Atk Pixilate Mega Gardevoir Double-Edge vs. 252 HP / 252+ Def Eviolite Chansey: 195-229 (27.6 - 32.5%) -- guaranteed 4HKO
It's a matter of Want vs Need, where she wants to include a fully physical move, but needs to support her main cast of Hyper Voice/Moon Blast, Focus Blast, Shadow Ball, Thunderbolt, Psychic, or whatever else you want her to have. Psyshock lets her stack the special and physical investments together to let her improve her speed or bulk, and it receives whatever Calm Mind boosts that may come, whereas any physical boosts will have to be passed in.
That's not to say that Double-Edge is necessarily a bad idea, but it's a situational move itself. The first real advantage that Double-Edge has over Psyshock is in Umbreon, who has the highest Special Defense of a Dark-type aside from Mega Gyarados, and is still 2HKO'd by a fully invested Hyper Voice. As for Gyarados, I'm not really sure what Mold Breaker does to Pixilate, but she shouldn't be staying in against it for too long anyway.
Aside from that, Double-Edge would only really be viable on Showdown, because she learns it from a FRLG tutor, and trying to breed a 5 IV ralts in BW2 is a hellish experience; I don't particularly care to try to breed something comparatively viable in the Emerald era, when Psyshock can be put on a nigh perfect ralts after a few hours in XY. In reality, this is likely a very minor point, what with Showdown being available and all, but it can still be troublesome for breeders.
252+ SpA Mega Gardevoir Psyshock vs. 252 HP / 252+ Def Blissey: 312-367 (43.6 - 51.4%) -- guaranteed 3HKO after Leftovers recovery
0 Atk Pixilate Mega Gardevoir Double-Edge vs. 252 HP / 252+ Def Blissey: 265-313 (37.1 - 43.8%) -- 99.8% chance to 3HKO after Leftovers recovery
252+ SpA Mega Gardevoir Psyshock vs. 252 HP / 252+ Def Eviolite Chansey: 228-268 (32.3 - 38%) -- 96.3% chance to 3HKO
0 Atk Pixilate Mega Gardevoir Double-Edge vs. 252 HP / 252+ Def Eviolite Chansey: 195-229 (27.6 - 32.5%) -- guaranteed 4HKO
It's a matter of Want vs Need, where she wants to include a fully physical move, but needs to support her main cast of Hyper Voice/Moon Blast, Focus Blast, Shadow Ball, Thunderbolt, Psychic, or whatever else you want her to have. Psyshock lets her stack the special and physical investments together to let her improve her speed or bulk, and it receives whatever Calm Mind boosts that may come, whereas any physical boosts will have to be passed in.
That's not to say that Double-Edge is necessarily a bad idea, but it's a situational move itself. The first real advantage that Double-Edge has over Psyshock is in Umbreon, who has the highest Special Defense of a Dark-type aside from Mega Gyarados, and is still 2HKO'd by a fully invested Hyper Voice. As for Gyarados, I'm not really sure what Mold Breaker does to Pixilate, but she shouldn't be staying in against it for too long anyway.
Aside from that, Double-Edge would only really be viable on Showdown, because she learns it from a FRLG tutor, and trying to breed a 5 IV ralts in BW2 is a hellish experience; I don't particularly care to try to breed something comparatively viable in the Emerald era, when Psyshock can be put on a nigh perfect ralts after a few hours in XY. In reality, this is likely a very minor point, what with Showdown being available and all, but it can still be troublesome for breeders.