Maybe we should be asking why those other kid’s shows don’t have an adult fanbase. All I’ve ever seen of MLP was the '80’s incarnation, and that was no more or less a toy commercial than GI Joe or Transformers back then.
MLP has been a part of the pop culture landscape long enough now that Bob’s Burgers did an episode making fun of the fandom, and one character on Teen Titan’s Go! watches an MLP type show within the show.