Here’s the secret origin of Black Panther’s massive success: it was culturally relevant, and it felt like an event. You have either of those, you have a hit. You have both and you have a massive hit. No further analysis needed.
The huge difference between this and, say, Justice League is that Justice League didn’t feel like either of those, and was certainly not talked about like them. The other major bonus is positive buzz itself. In these blockbusters it really does pay, in this day and age, to have a positive critical consensus. These Avengers movies have consistently delivered what critics want to see from superhero movies. Wonder Woman did that, too, obviously. At this point you don’t need a grand conspiracy about it; critics like what critics like, and like it or not, they will always have certain expectations for certain genres. Not a single one of these has ever been nominated for Best Picture at the Oscars. When Dark Knight missed the cut, it resulted in a revolution in how the nominations were listed. Black Panther admittedly looks like the next superhero movie with at least a shot at a nomination, but that remains the most intriguing bit of speculation for 2019. Here in 2018 we can look at the history and make an educated guess and suspect that won’t happen, because the Oscars don’t generally like genre movies, no matter their level of acclaim. Worth considering, in the grand scheme.