I think part of it is because their last tour, for whatever reason, was cut short. There’s also the fact there was no culminating *thing* for closure, like a recording or something to commemorate that, not that it was something that was required.
Also, I feel fans have begun to have a whole expectation about artists celebrating their milestone anniversaries in a big way for every major milestone of their career. (Example: People being upset that there was only a video from Britney’s team commerorating the release of BOMT, and not a whole promo campaign behind it.) They missed their 20th anniversary, but maybe fans still expected something and they’re clinging to this as it?