Black Panther did tackle weightier themes - straight on - and is far more auteur than most Superhero movies. It was also in the shadow of controversy regarding the Oscars and racial politics, which probably effected some of that decision.

Endgame does things well, but it's just another big-budgeted action movie at the end of the day. It certainly wasn't made with the intent of trying for an Oscar, just to make money and push the Marvel story and brand further.