Why isn't Northstar more popular...?
Because "I'm gay" is pretty mediocre a beginning for the start of a heroic fiction, and it the way its been used thus far it tapes over any other fact about the character.
In fact this very thread is an example of the why he's not more popular. The general opinion or idea that
"Representation is vital in today's climate"
"Yes, yes, it is known" - game of thrones but... to probably about half the people it really isn't.
What's vital is an interesting, and well flushed out character with relatable(or at least understandable) motivations, challenges, and foibles. Dare I say? Villains.
People will buy that, people will care.
I tend to point at "The Midnighter" in wildstorm comics since I have every issue of the end of stormwatch and The Authority. (not the mess that dc put out
post purchase) as a good example.
Him being gay wasn't the driving factor of the book it was something he WAS to be sure but NOT what he was "doing" as character at any given time.
Just my initial thoughts.
EDIT: Some other things.
Literally type in "Northstar Marvel redesign" go with any of those looks. Anything but the last thing I saw him in during the X books.
Secondly, give him a slogan or catchphrase. I think... on some part he's a beloved national treasure of Canada. If not their peacetime Captain America go with that.