I think they did it the way they did because they wanted something to capitalize on the success of Black Panther but didn't want to have to make a completely different series to do it, so they re-structured Assemble to make it work.They tried to capitalize on the movie but failed to capture much of what made it the phenomenon that it was. Sadly, this was the common MO of Avengers Assemble.
That last season should have been either Avengers Assemble or a completely different show centered entirely around Black Panther and his world. I don't see why they couldn't have done two series; maybe a limited Black Panther series and a final season for Avengers Assemble that might satisfy viewers who actually stuck with the show through the previous four seasons. I'm not sure that it even makes sense, in canon, for Black Panther, a character who the show never solely centered around for more than one or two episodes, to now be the star that the show revolved around for the final season. That would be like the Question being the main character of the final season of Justice League Unlimited.
I'm not sure if, production-wise, they could have made a limited series or a brand new cartoon. When this was produced, Marvel Animation was probably heading towards closure at this point.
T'Challa was probably the most prominent new character in the prior seasons of Assemble, next to Ms. Marvel, in my opinion, so I don't think the shift in focus came completely out-of-nowhere. He was the main leader of the New Avengers for Secret Wars, too.