No reasoning was given, but it’s likely because the Classic transfer tokens could be also used for Anniversary realms so they disabled it entirely.
Yes and no, it was very late Vanilla like some time before TBC that cross-type transfers were enabled. The Era/SOD could transfer between the types too some while later/directly. I think after Blizzard re-enables transfers again for Anniversary (The Classics) you may be able to do this but it may take awhile.