Think the best answer is that people were just tired of Dragon Ball at the time. GT didn't have a 17 year gap between it and the original, the series aired right after Z ended. I can understand the sentiment people probably had after Z ended. There was now suddenly even more Dragon Ball and it didn't change at all and even worse it borrowed more from early Dragon Ball. (There's this weird disconnect between people who started with Z and people who didn't, the former absolutely hates everything before Raditz and hate it when it's acknowledged.)
I think Super's success comes from how much time passed between it and the original (even GT), it left enough time for people to become nostalgic of the series, people were born and raised on Dragon Ball, never watched anything else. And they desperately wanted more. I think if Super released just after Z ended it'd have met an even worse fate than GT, considering it sucks dog shit.