I honestly think it's just that the show has gotten old, and the generation that grew up watching it are mostly adults and older teens now, and for most people in that age range, they've stopped watching cartoons made for younger kids. There's also the quality of the show in general. While I believe that the 2nd movie, and seasons 9B-12 have been a MASSIVE improvement compared to seasons 6-8, it doesn't change the fact that that's still people's general impression of the show now.
Most casual viewers (which sadly makes up the majority of people who watch cartoons aimed at a younger audience) don't really care enough to give the newer seasons a chance because either A) like I said, they've just grown out of it and don't care enough about it anymore, or B) Spongebob has simply gained a status of infamy for everything past the first movie.
Infamy as a TV show, especially with how badly post-movie Spongebob has received it, is really hard to recover from. It's the same reason why shows like The Simpsons will sadly never be seen in the same light again as their golden age, even if they manage to improve and somehow become better than their roots. Once the general public makes up their mind on something, it tends to stay that way, forever. It's popular to hate Spongebob, and most casual viewers stopped watching somewhere around seasons 5-8, and with subjectively some of the worst episodes of the show being your last impression on it, combined with the hate boner the internet has for modern Spongebob in general, it makes sense why most people's opinions would stay the same and they wouldn't want to give the show another chance.
Also like I said, the show's just gotten old in general. While like The Simpsons, I believe Spogebob is a show that can live throughout and entertain multiple generations, we're kind of in an era now where most children's entertainment comes online through internet personalities and video games. Spongebob just isn't cool or relevant with the kids anymore, and the adults who grew up watching it have moved on and have a general distaste for the newer episodes. Seeing Spongebob on every corner I'm sure helped the show feel very over saturated and watered down too. He's essentially become the Mickey Mouse of Nickelodeon.