Pokémon: Gotta Catch 'Em All!

Here I go along, catching a shiny, double marked Goomy in Pokemon Scarlet, and my inner child is thrilled. I love Goomy and all pokemon connected with it. Didn't release in the greatest of times (really should have been poison/dragon given its release with the Fairies in gen 6, but that's a different point to make), but I still love it. It's goofy, cute, and REALL bulky for some reason. So I go about evolving it, which isn't that hard, so now I have my Goodra.

I go to teach it some moves, EV train it, the works. TM it Dragon Pulse, Body Press, and then something on my move remembering doesn't pop up. Goodra can't learn Acid Armor. Now I'm thinking "okay, that's a little strange, maybe I need to level it up. I max it out. Nothing. I check Bulbapedia, and it states "Acid Armor can only be learned by Sligoo.

So I check Sligoo's page. I love it to bits for being so derpy, but why on earth is it a middle evolution that can only have the move heart scaled? I would have been more forgiving if it had just been a level-up move that I forgot to get, if it was like level 60 or something, but having a middle stage pokemon locking it out of its best defensive move because some trainers didn't know that only Goodra can learn Acid Armor if you go in to remember a move in its awkward teens is a bit of a sucker punch.

So I ask to you all, what are some of the theories y'all have for why Goodra can't learn this move? Hisuian makes sense, as it uses Shelter to guard itself with its shell, but Kalos Goodra doesn't have a shell, it literally has armor made of gooey acid. So, as I wish you all a great day, I'll ask this as I asked in the beginning: Why only Sligoo?