Most popular sport or highest popular ... why cant it be highest popular
Because "highest" is an adjective and good to modify nouns, such as "popularity".
"Popular" is not a noun but an adjective and needs to be modified with an adverb. Just like you can't say "high popular", you can't say "highest popular" either.
That's why you say "high popularity" and "highly popular" and never the other way around.
You could say "most highly popular" if you really wanted. It would sound a little odd though.