Shame that there wasn't money/time/interest to port one or two of the collections to the Switch.
I think it was mostly a money issue. I suspect Sora was in the works for Smash for some time, and the ports were part of that discussion. It's especially odd though since, as the article notes, the first game contains a GBA remake, a DS remake, a PSP remake, and 2 PS2 games that were originally released on the PS3. The second game was on PS4, but again, it's primarily a 3DS remake.
The final game being PS4 native probably meant that it was always destined for the cloud, and if Square was going to set that up for KH3, it probably made more sense to just save the money and put the other games on there as well.