It's because the "form" has absurb stats.

Unless a Pokemon is a legendary, pseudo-legendary, or Mega, Game Freak doesn't allow it to have a higher base stat total than Arcanine without an ability that hinders it.

I don't see what's wrong with it being a form. I would've liked it to be an evolution, yes, but just seeing a tiny mackerel turn into a thing bigger than Gyrados is amazing. I think the part about it losing its form at a certain area of health (Idk if 50% was confirmed or not maybe it's lower) is to make people use it well. If it was to be used in competitive, it would need Pokemon before it to set up incredibly powerful defense stats and then baton-pass.

Honestly, I wish it activated when it was BELOW 50% health, rather than above. Like, sort of a "I'm really fucked here, guys, can you come help?" sorta thing.