Anime- How come certain characters are considered main characters?

You can't have a story where too many characters can be considered "main". There must be specific protagonists for the story to follow, or the audience would become disinterested with the shallow overview of a bunch of characters rather than on specific one. It also helps to have a main character for the audience to cheer on, since when the protagonist gains victory against all the odds is more satisfying for the audience than a group of people doing the same. It withdraws from the singular achievement if there are many people to share it with. That being said, you can have anime with a group of characters that are considered the main ones and nobody gets that much air time over the others. There will always be one or two that do stand out though, this happens in lots of animes based on club activities where every character has a significant role to play and the groups success is dictated by their overall performance rather than one character's. An example of this would be "Love live" and pretty much any other idol anime.

Its just what it is - ANIME PHYSICS

ANIME IS AWSOME