The reason is due to how the possibility to have HA works in SwSh (in order to be able to have HA, the pokemon must be able to spawn as purple raid in same den.)I’ve been trying to catch a Frosmoth with its Hidden Ability but serebii says it only appears with Ice Scales in Sword (I have Shield btw).
I would assume in Shield there's no den that can spawn Snom/Frosmoth as purple beam, thus HA isn't obtainable in that game via raid.