Because those others only had one-shot cameos, most of them having no narrative effect on Arrowverse storylines, while Earth-90 and its Barry Allen played a significant narrative role in two major storylines, as well as having a more indirect ongoing influence through the Earth-1 doppelgangers of characters like Tina, the Trickster, and Bellows.
Clearly, the intended implication of Crisis was that every DC screen universe (and comics universe, if you believe the comics tie-in special) is part of the same multiverse as the CW shows. So if you include every crossover world, then "Arrowverse" becomes synonymous with "DC in general" and becomes uselessly broad. It's more logical to save it for shows that have a meaningful, ongoing story connection to each other, rather than one-shot cameos. If and when Stargirl starts crossing over repeatedly with the other shows (which it probably will going forward), or if Earth-96 Superman or Lucifer should show up again, then we can add their worlds to the Arrowverse roster.
The difference is that the Earth-90 characters and their Earth-1 doppelgangers were played by the same actors. They were "true" doppelgangers rather than just namesakes. We saw multiple Superman and Batman worlds in Crisis, also including Smallville and Batman '66 and '89, all of which had their own different versions of characters also appearing in the Arrowverse, but those characters are only implied; they didn't actually appear onscreen in an Arrowverse series. Only Flash '90 had the same actors appear in the Arrowverse as the same (or equivalent) characters in recurring roles.
