Because most of what I see labeled as "street photography" is crap. Somebody walking around pointing a camera at things and pushing the shutter. No connection to the subject, very little concern for background and composition, just, "Hey, look, I take pictures on the street, aren't I edgy."
There are great street photographers, like Mary Ellen Mark, Bruce Gilden, Vivian Maier, and I do enjoy looking at their work, but so much of the rest in pretentious crap.
Street photography is one of the easiest forms of photography in terms of cost in time and equipment. Anyone can get a camera, walk out their front door to the street, and start snapping away. Which is why there is so much street photography done, and just like any form of photography, there are few real artists, so the majority of the images are bad street photography. If fashion photography or sports photography or (pick some other form of photography) were as simple to do, you'd see a lot more of it, and just like street photography, most of it would be crap.
