Black Panther just ain't an interesting character. When I read comic books back in the day, he was never a character that I would get hyped about.
The character won't be able to carry a movie on his own. Same reason why Black Widow or Hawkeye will never get their solo movies. Those characters aren't strong enough to carry movies on their own.
Guardians was a group of characters, that's why that movie worked even without a high profile character. Imagine if it was just Starlord, no one would want to go see that flick.