These are arbitrary, by and large senseless, and serve only as a developer’s convenience.
If they continually region-lock a toy, they don’t have to bother coming up with new ideas later - just do the exact same thing, just in the new regions. But only in the new regions - can’t have anyone getting any ideas that they won’t have to find/grind this all over again next expansion. It’s the same with the persistent Augment Runes, and, honestly, if they could get away with more zone/level locked “rewards” they probably would. They’re already pushing that envelope to truly obnoxious, awful extents already.
This “only SPESHUL MOUNTS WORK IN THE MAW” BS is only the beginning - mark my words.