First off, usage doesn't have anything to do with the rankings. We don't care how good a mon is at beating bad players, we care how good it actually is.Cause Im a goose I forgot how to quote, but that's ok.
You said how Shed and Chansey beat Dialga. Well, if you didn't see my set has Roar. Roar phazes out Shed into a mon that can get killed by Doom Desire.
+3 252+ SpA Dialga Doom Desire vs. 4 HP / 252 SpD Eviolite Chansey: 357-420 (55.6 - 65.4%) -- guaranteed 2HKO Chansey literally dies I don't know why you mentioned this at all especially with sub. They have to run Whirlwind which I'm not gonna say is uncommon cause that'd be a lie. It's on about a quarter of Chansey in the tier. So 3/4 times Dialga beats Chansey.
Garchomp isn't exactly the biggest threat in the meta, 6% usage means I'm on average going to see this mon on 3/50 teams on the ladder. Not to mention that 4% of these Garchomps are actually running Dual Chop, and assuming you are talking about the Adaptability set which has the power to break the sub, that's only seen 3% of the time. I don't think Garchomp is going to be a problem for Dialga in the current meta.
Magic Bounce/Unaware needs to be played very VERY carefully in order to beat Dialga, assuming you have both on one team. If you just have Unaware, Dialga Roars out and gets a kill, if you only have Magic Bounce, Dialga sits behind a sub and kills with a +3 Doom Desire. If you do have both, its a 50/50 chance that you play the Dialga correctly.
"it doesn't even hit particularly hard..." What? This mon is one of the hardest hitters in the metagame. +3 252+ SpA Dialga Doom Desire vs. 252 HP / 8+ SpD Assault Vest Registeel: 148-174 (40.6 - 47.8%) -- guaranteed 3HKO Ass Vest Regi can only click U-Turn to go to something that may or may not be able to break the sub. A lot of the things I'd agree with you if it was only on paper, except in actual practice Dialga performs stellar.
Both Regi and Slaking struggle with Imposter, that's what team support is for. Regi needs an Aegi or a Shed with Recycle and Imposter isn't too much of a problem anymore. The -ate is actually a huge deal, especially when the opponent is relying on Ray being able to revenge kill, or Diancie for that matter. And be careful when you click Shift Gear so you don't need to sack something to Imposter.
You didn't acknowledge the Spider Web set which would be the main selling point of it being better than Metagross. It traps the -ates and can kill or stall them out. Patience is key when playing this set, and if anything its on the same level of Metagross.
And why should Kyurem-B drop exactly? It's main set is SD right now which is honestly an incredible set. SD / Espeed / Steel Killer / Filler. This set is crazy good at taking down so called counters. I don't use it personally but playing against it is difficult, despite having -ate counters. I don't see a reason for it to drop.
It's weak compared to the strong mons- latios, PDon, MRay, MBlaze, MMY
+3 252+ SpA Dialga Doom Desire vs. 0 HP / 0 SpD Mew: 696-820 (204.1 - 240.4%) -- guaranteed OHK
252 SpA Sniper Mega Mewtwo Y Psycho Boost vs. 0 HP / 0 SpD Mew on a critical hit: 699-823 (204.9 - 241.3%) -- guaranteed OHKO nuetral Mew
252+ SpA Choice Specs Aerilate Mega Rayquaza Boomburst vs. 0 HP / 0 SpD Mew: 625-736 (183.2 - 215.8%) -- guaranteed OHKO unboosted
+3 252+ SpA Soul Dew Mega Latios Spacial Rend vs. 0 HP / 0 SpD Mew: 783-922 (229.6 - 270.3%) -- guaranteed OHKO stronger
+3 252 SpA Life Orb Sheer Force Mega Mewtwo Y Psychic vs. 0 HP / 0 SpD Mew: 840-990 (246.3 - 290.3%) -- guaranteed OHKO stronger
252+ Atk Choice Band Mega Blaziken V-create vs. 0 HP / 0 Def Mew in Harsh Sunshine: 849-999 (248.9 - 292.9%) -- guaranteed OHKO stronger unboosted
+2 252+ Atk Primal Groudon V-create vs. 0 HP / 0 Def Mew in Harsh Sunshine: 1236-1455 (362.4 - 426.6%) -- guaranteed OHKO much stronger after a shift gear
Dialga also takes 3 turns to do damage, so anything it fails to OHKO it can't KO at all. It has a low DPT, is vulnerable to actual attacks, and just loses to useful moves like whirlwind, heart swap, encore, taunt, and attacks.
I didn't acknowledge spider web because spider web is useless, on account of it losing to the ates it's trying to trap (maybe it beats some diancie? I mean I guess)
So KYub should drop because it's outclassed. MRay can do any set it has better, and the fact that it doesn't should say a lot about how "useful" KYub-Bs niche is. Literally the only reason to run KYub is to not die to other refridge/have a slightly better matchup vs Ray, which is self defeating.