I'm going to swoop in and steal this from Cameron H but he'll come on later and articulate it a bit better.
For me at least I think the biggest thing that makes the original and this different movies is the unoriginality of the sequel. The sequel while a different story hits almost all the same beats, gags and moments as the first but a lot of them to lesser degrees. Just think about how many things are the same? Released from prison to start, go on a quest to put the band together, recruit their former members who are now doing other things, get a mismatched gig and win the audience over, race to get to the big show, big final number the crowd loves. That's not even mentioning the various gags and moments they repeat like James Brown in Church and seeing the light, Aretha Franklin mad at her husband and singing a song to tell him off, making white nationalist mad and have them chase after them, etc. Just so much of this movie repeats the same notes and doesn't really do that much new or different with them. I hate to bring it up but take The Force Awakens for example. It hits so many notes of the original Star Wars and the original films, but the spin they put on it makes it more original and fresher. It is not just a simple rehashing of it. So with the original Blues Brothers the gags and story and fresh and original and therefore have more life to them. In 2000 they are just going through the motions and it doesn't have the same life. And when you've seen that all before it is boring for you, and when the actors and director have done it all before it is just as boring for them which in turn you feel and get more bored.
On a side note, 2000 looks terrible. Bad lighting and clearly shot on sound stages while the original was shot in real locations and little things like that really affect the mood and vibe a film gives off too.