There is a Dojo just a mere 10-minute walk from my home. I hope to suppliment my current weekly training at my own club (which is a good 30-minute drive) with regular training at this nearby Dojo, but cannot simply because it is run by a non-affiliated club.
I am not concerned about rank, grading or differences in styles (willing to wear my white belt all over again), but I am still not welcomed.
How is that connected with unification? This is an issue of teachers methodology and not of
Different teachers teach in different manners, for most of them, a student saying "but in XXX dojo they do it the other way" would create an interuption they are not willing to accept, be XXX in the same organization or in another one. Some of those teachers would not know to explain the reasons for the way taught by them, others, are afraid to admit another variation might be better, and others would not be willing to find a disruption in class becausse some students are trying the other varyiation and have problems.
Some teachers also believe it is not in the best interest of their student to learn the same technique with varying emphasis at the same time. They consider it to confuse te student and hamper is progress. Such a teacher may refuse to have his student consistently learn a very similar M.A. in another place
Some teachers would consider such a state as putting them in a continous popularity contest with another teacher, and thus hampering their methodological considerations (the students should practice the same techniques for the nth time, but then he will think of my lessons as boring compared to ...)
A good teacher invests in his studens. Some may wish to feel they are the ones responsile for his progress rather then share the fame.
Lots of reasons, all of which have to do with humans, not with organizations.