Spoiler Warning: Spoilers concerning the Infinity Stones follow for Black Panther. If you haven't seen the movie yet, bookmark this and come back when you have.
Now that Black Panther is officially in theaters (and officially killing it), the next film in the Marvel lineup is Avengers: Infinity War, which will find the Avengers attempting to stop Thanos from acquiring all of the Infinity Stones. So far, we've seen five of these stones introduced across the entirety of the MCU, and several fans theorized that the final stone -- the Soul Stone -- would make its debut in Black Panther. As people who have seen the movie know, that wasn't the case, but the director and co-writer Ryan Coogler has a pretty good reason for wanting to keep the stone out of the movie. Here's what he said:
Due to Black Panther being the final movie before Avengers: Infinity War drops at the top of the blockbuster season, some thought that the movie would have to be introducing the Soul Stone to help set up for Marvel's upcoming epic. Instead, Black Panther is as stand alone as they come and only focuses on telling its own story rather than trying to service another. So, the Soul Stone is still MIA and it sounds like no one making Black Panther was too concerned with working in the cosmic stone, to begin with.
Ryan Coogler told IGN that the Soul Stone wasn't included with the movie because Wakanda already had a MacGuffin in the form of Vibranium, which can basically do anything. Coogler felt that Vibranium was special enough and it probably wouldn't have helped the film to take up more time to explain an entirely different supernatural item. Coogler goes on to say that there was never any pressure to include the Soul Stone and that Marvel Studios was "never really interested" in including it either.
I think that Marvel and Ryan Coogler made the right call on this one. Black Panther was much better off telling its own story and introducing its own world than having to take precious screentime for a shoehorned reference to a movie that comes out in less than four months. Besides, Marvel doesn't have the best track record when it comes to this sort of thing. Or need I remind you of a certain magical hot tub? Plus, it creates more mystery for Avengers: Infinity War and how the final stone will play into the grand scheme of things.
You can see a bunch of great stuff (but not the Soul Stone) in Black Panther, which is out in theaters right now.