Because they are similar, not identical. I have Dan ranks from the KKW, the ITF, and the MDK, so I'm one of those you're talking about.
Honestly, some of it is availability. I grew up moving to a new military base every 3 years or so. And at that time, ITF schools were easy enough to find. When there was none, I trained in other systems, including systems totally unrelated to TKD. Later in life, ITF schools were more difficult to find. Hence the KKW and MDK.
But there is certainly more to it than that. There are small, sometimes very subtle differences in the way these similar systems approach things. There are differences in how specific things are applied. There are differences in what aspects of the Art are stressed. Much of this can be seen even within different schools that are nominally part of the same system. And that is good.
Personally, I think that only training in one school, under one set of instructors, is less than ideal. People who do so are apt to become members of the "one way" school of thinking. This can slow their development and understanding.
Take the way people look at the techniques in forms.
@skribs, this is a question specifically for you, as the OP, but it would be good to hear from others.
In Palgwae 8, there is a movement during which you step to the side and throw both a rear elbow strike and a punch of=ver the same shoulder.
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What is your understanding of the application of this movement?
As with other movements in forms, students are taught that the movement is used for "X". But what I hope to show here is that "X" will vary depending (in part) on those differences I mentioned above.