The only other traditional Frozen short, Frozen Fever, came out in 2015 attached to the theatrical release of the live-action Cinderella.
Perhaps plans were to release this in theaters attached to Reya and the Last Dragon, which we originally were going to see in November.
It makes one wonder if there was a plan to release Once Upon a Snowman theatrically, but that those plans were scrapped following the shakeup of the release calendar.
It’s also possible that this was always eyed as a Disney+ release.