I think that happened due to the way Coogler and his writing partner structured the movie. They placed so much of the emphasis building Wakanda's mythology and not on T'Challa himself, that so many of the film's fans adored the supporting cast. For example I was on twitter and a thread popped up into my feed asking for people's favorite MCU character. The thread at the time had over 60 replies and I scanned it to see how many people picked T'Challa. At the time only 1 person chose T'Challa as their favorite character. More people chose Okoye, 3 at the time, in the tread more than T'Challa. I was really shocked at how many chose Bucky but that could be recency bias due to the new show. Now this was just one twitter thread but I think it shows why so many believe that the franchise does not need to recast T'Challa.
One of my friends and I were talking about the BP movie and he watches all the MCU movies and read a few graphic novels. He never really read any BP comics. Watching the movie when T'Challa and Killmonger faced off at the waterfall, he said he felt that T'Challa was the underdog because Killmonger was a MIT grad, a Navy Seal, CIA operative and had prepared for that singular combat moment. I told him it was not true and then went on to explain T'Challa's training but none of the MCU movies had done so. He did not know how well trained T'Challa was because the film did not put emphasis on it. He was a bit shocked that the film did not show and talk about T'Challa's training. Also so many of the other characters had dynamic personalities that popped on film that the MCU and Coogler believes they do not need T'Challa. I have so many friends that said after Boseman died, they should just give the mantle of BP to Shuri since it happened in the comics. Most do not know that in the characters history, Shuri was only BP for what? Maybe 2 years in the 60+ years of BP history? They do not know the importance of the character because the film place more emphasis in building the mythology of Wakanda and the side characters and not T'Challa himself.