Ok, this has been unanswered, for....a very long time. First, gardevoir has a horrible HP stat of 68. Second, "tanks" really aren't common in vgc. If you have a "tank", it has to fulfill other roles. This is why chansey is bad in vgc: It only tanks hits. It can't dish out damage or do another thing. It does have some alright support moves in taunt and helping hand, along with trick room, but all of those moves have better users. For taunt, tapu fini or whimsicott have that covered. Whimsicott also gets helping hand. On top of that, trick room is more consistently set by cresselia or even comfey. Heal pulse is useful, yes, however, that isn't something that can propel gardevoir into viability. Plus, gardevoir isn't even unique in the combination of Heal pulse, Trick room, Helping hand and taunt: Gallade and the gothitelle line get it. So, in summary, gardevoir has a bad hp stat, tanks aren't common in vgc, it does not get many good support moves and role compression is not a factor.
On top of that, it is weak to many common attacks. For example, it is weak to a shadow ball.
252 SpA Life Orb Spectrier Shadow Ball vs. 252 HP / 252+ SpD Gardevoir: 242-289 (71.1 - 85%) -- guaranteed 2HKO
It cannot take hits from venusaur either.
252+ SpA Life Orb Venusaur Sludge Bomb vs. 252 HP / 252+ SpD Gardevoir: 229-273 (67.3 - 80.2%) -- guaranteed 2HKO
In comparison, if we look at cresselia, one of the only "tanks" in vgc, it can take hits a heck of a lot better.
252 SpA Life Orb Spectrier Shadow Ball vs. 252 HP / 252+ SpD Cresselia: 221-265 (49.7 - 59.6%) -- 99.6% chance to 2HKO
Now, yes, this is still a 2 hit KO, however, look at the percentages. It is shown to be that cresselia takes it better, so the second attack coming at it might not ko it, while the one coming at gardevoir will.