It might be the case Matthew, but I have a feeling it may be to do with several factors.
One reason might be to do with the words themselves. My feeling is that for BrE speakers anyway, "owner" is a more instinctive term to use than "proprietor", which is longer, has more syllables, and is slightly harder to pronounce. Since the two words mean exactly the same thing in English [update: I mean in the context of this topic thread, where people have a choice between the two words], why would people choose the longer word when a shorter, simpler one, will do instead.