I would love to know which supports favor guardian because I literally can't find a single one that picks it over Aftershock.
Thresh: https://na.op.gg/champion/thresh/statistics/support 88% for Aftershock and a higher winrate
Pyke: https://na.op.gg/champion/pyke/statistics/support Doesn't even take guardian so not applicable.
Blitz: https://na.op.gg/champion/blitzcrank/statistics/support 66% of the time is taking aftershock and only losing a little bit more than 1% on his winrate for it; he only takes guardian 16% of the time.
Taric: https://na.op.gg/champion/taric/statistics/support 58% of the time is taking aftershock; his winrate with both guardian and aftershock are good but he only takes guardian 20% of the time.
Leona: https://na.op.gg/champion/leona/statistics/support Doesn't even take Guardian
Alistar: https://na.op.gg/champion/alistar/statistics/support Also doesn't even take Guardian
Galio: https://na.op.gg/champion/galio/statistics/support no Guardian
Nautilus: https://na.op.gg/champion/nautilus/statistics/support A continued lack of guardian
Braum: https://na.op.gg/champion/braum/statistics/support Is the first and only support I've found to take Guardian more predominately than aftershock.
Rakan: https://na.op.gg/champion/rakan/statistics/support This is the second. The thing they have in common? Both are incredibly under powered and sitting at 47 to 48% WR's. This is not a good case for your point.
Gragas: https://na.op.gg/champion/gragas/statistics/support Aftershock strikes again.
Tahm Kench: https://na.op.gg/champion/tahmkench/statistics/support He takes guardian by virtue of none of the other runes fit him and, like Rakan and Braum, is sitting at an abysmal winrate; 42%.
Shen: https://na.op.gg/champion/shen/statistics/support Aftershock
Poppy: https://na.op.gg/champion/poppy/statistics/support Aftershock.
There are very few supports who prefer guardian over aftershock and all of them are sitting at sub 50% winrates. They may fill different slots but this is soloqueue; no one is going to necessarily pick for what is right for the situation. Furthermore, the fact that the ones that primarily prefer guardian are all sitting at sub 50% winrates does indicate that the rune itself isn't very powerful.
Furthermore, Aftershock gives 70 extra Armor and MR at level 1. I don't understand how that couldn't possibly benefit tank supports. Add on to the fact that the CD is a flat 35 seconds compared to Guardian's is 70 seconds, yes, 70 seconds, at level 1. And it only gets to a 40 second CD late game. Considering how often you can accidentally proc it, that is absurd to me.
Guardian is extremely weak and completely overshadowed by aftershock as proven by the fact that the class that should want to take it is instead taking Aftershock. And the ones that do take Guardian as their primary are doing godawful.