I think there is too much separation in the world in general.
As children, we see the world in black and white, but with maturity and experience, eventually begin to see it in shades of gray. However, this "black & white first" seems to continue into adulthood, where the first time we are presented with a new concept (such as martial arts), in order to understand it, we see it in terms of black and white. In that case, we'd want to know: is this art internal or external? Is it for fighting or health? Should I embrace it completely or reject it outright?
I can only assume that this attitude is propagated by teachers and promoters either because they never grew beyond a black & white understanding of their art or they pander to the masses (well, you gotta have students) by promoting the black & white understanding so the masses can understand. For example, if a prospective student asks "is your art internal or external?", and the teacher says "it's both", the student may not believe the teacher knows what he's talking about, or may think the teacher is lying just to get the student.
It may also be that, given that an art starts one way and ends up the other, making a distinction and naming an art "internal" or "external" best serves the students who only expect to proceed halfway through the art and stop when the emphasis changes. I think most people see themselves as "soft" or "hard", and when asked to approach the opposite, tend to balk. So they quit. Hmph. No wonder there are so few masters?