I have come to the conclusion that many people actually cannot see what, for example, Cartier-Bresson was doing. If they have any sensitivity to art they may perceive that there's something going on, but they lack the vocabulary of design and composition to really see it.
Since most photographers today are in fact camera enthusiasts, sensitivity to art is not really a given.
So, for various reasons, because they cannot articulate what is different, they lump all black and white pictures of people in as 'street'