The speculation I’ve seen that (unfortunately) makes the most sense is that it’s a lot of work to create a prologue tutorial for each class and as they kept adding new classes (over 20 now in the Korean version) they got tired of the expense/effort involved in adding the new ones.
If they’d been smart they would perhaps have realized that the NA release being limited to only the smaller set of older classes was a good opportunity to reintroduce them for a while and then they could pull them again once the T2 stuff and classes were released here in a year of so. I suspect it’s one of those things they never really thought about. It’s a shame because they are really good class/game intros. Missed opportunity all around.