I have a feeling when I ask myself this question that perhaps it has something to do with the well known phrase “the grass is always greener on the other side of the fence”. This is especially true when watching long anime (mostly Shonen Jump), because I almost feel like I’m married to the main character; an arranged marriage I had no part in.
What makes a character great? Fully fleshed out characters with likable personalities would be my best guess; but when it comes to anime that is not all. We also like interesting super powers, unusual abilities, or normal people stuck in unusual situations. I sometimes feel that mangaka are forced to make their mains a certain character archetype, so that the audience can easily relate themselves to them. Yet for side characters they don’t have to follow any of these “rules”, so they go all out.
Shinigami Captain’s bankai and compare it to Ichigo’s, do you see how badly he was shafted? I guess that’s why Kubo threw in all the crazy transformations for him.
This is only a problem when side characters are forgotten from the story; but usually this isn’t as big of a deal as I made it out to be. As long as anime and manga exists, this will always be a problem. So the only way to fix it is to not make it a problem at all. Enjoy it for what it is, and not for what we want it to be. Who really wants to watch a Kamina anime anyways?