This particular adjective isn't used in the comparative or superlative forms. Perhaps someone else can provide a more academic explanation, but this appears to be in the group of adjectives that are "binary" (either it is, or it isn't) and it doesn't make sense to say they have degrees. Off the top of my head, these would include things like pregnant, on/off, plastic, and alive/dead. Similarly, the synonym "primary" isn't comparable because either something is the first, or it is not.

the word "main" does not have a comparative or superlative form.

There is not a comparative or superlative form of the word, "main" Did you mean to say "many"? If so, it is: Many, many more, and most.