You're misunderstanding what the movie explained. Wakanda stayed unknown until the last few years and T'Chaka revealed the country. But he made sure to make everyone think that Wakanda was this backwater 3rd world country not worth investigating or lots of attention. The point of BP was T'Challa deciding that this was still BS and he unveiled the true power of Wakanda to the world. Nakia? Whatever she was doing was for protecting Wakanda's status as a fake 3rd world country. Stuff no longer necessary by the end of the film. He's starting to, but he's also willing to start sharing with other major countries to keep them from getting too curious as to what was going on in Africa and provoke any kind of international incidents. So to avoid international repercussions from them helping their neighboring countries. Because there's no magic cure and a downside to becoming a true part of the international world is that everything you do will attract attention. So might as well nip that in the bud by making yourself available to all the world instead of helping lesser countries and get the big guys mad at you for not wanting to work with them as well. They want their piece of the pie after all.