It's because they're not White Americans.
See since the NBA is an American product, a Euro player dominating and having success isn't going to resonate with most fans, particularly White fans. A White athlete will only go so far if White people are standing behind them.
Take Caitlin Clark for example, if she was a White Euro woman people wouldn't have railed (to the overwhelming extent we saw) behind her as much because they wouldn't be able to relate but, since she's an American-born White woman from Iowa representing what's commonly considered a “Middle America” state she resonated with lots of people especially White so she blew up culturally in the women's college basketball world.
Subconsciously people like to champion someone they can relate to and by that, I mean racially and ethnically. I'm many Black American NBA fans like Jokic and Luka from a talent perspective but there's no need to relate to them when many great Black Americans are playing in the league now.
The Great White Hope is usually made about an American-born White athlete defying the odds and dominating a sport/opponent. Luka and Jokic despite being White (in appearance) they're not American so they won't ever blow up on a cultural level like say Larry Bird, Caitlin Clark, Austin Reeves, Alex Caruso, and many other White athletes who were culturally celebrated in their respective sports.