Because he had a personality.
 
Grogar is obviously going to get his time in the spotlight for the finale, but barring that, he’s a paper thin plot device to break out/resurrect/unify the villains and nothing else.

He’s been around longer and was infinitely less boring

Someone keeps posting pictures of the movie

“I mean, come on, I was a G1 character! I should have more by default!”

 
Storm King: What? You mad, old timer?

You two have the same exact number of tags, 1.
 
However, he is tagged on more images.

Don’t worry, Grogar, you’ll get more tags soon. I just need to see if you’ll live up to the hype of being built up as the Big Bad of the whole show.

Matter of time before you catch him up.