Wakanda remaining behind was inevitable. Of the four Infinity Stones currently on Earth, three had been taken away by Peter's expeditionary team. The remaining one had to stay on Earth, lest some ill-intentioned people managed to gather all six Infinity Stones. Of course, aside from the Soul Stone, which was most likely in Ego's hands, another Infinity Stone was currently missing. That was the Space Stone. The Space Stone was inside the Tesseract. Thor had previously taken it back to Asgard and handed it over to Odin.