Why Black Panther's Surprise Cameo Didn't Happen Until The End Of The Movie

With Black Panther being the last Marvel movie before Avengers; Infinity War, many of us expected this movie to set the stage for the big team up with lots of references and set-up, and with the end of Captain America: Civil War showing us that the Winter Soldier was in Wakanda, he seemed like the most obvious Marvel character to appear. The main film, however, only makes a passing reference to him as a "broken white boy" but in the closing credits, we see Bucky Barnes with Shuri, having been apparently successfully rehabilitated. However, we also see that he's not quite 100%. Ryan Coogler tells the Empire Film Podcast that it only made sense for Bucky to be in this state, so sending him to help in the fight in Black Panther just wouldn't work.