I'm not fully convinced that Shuri -- as we've seen her -- can carry a movie as a lead. She works as a fun, comedic-leaning supporting character.
I'm also not sure she brings anything unique to the table, personality-wise, in the eventual future team-up movies. We've got the young, wisecracking, tech-literate bases covered already with Peter Parker. T'Challa supplied a uniquely stoic, noble energy to the lineup, and while he wasn't totally humorless, he was definitely one of the more serious heroes (by design, and that's a good thing).