A lot depends on how comfortable the photographer is in approaching people and the perspective they want for their images. People have historically used the 50mm lens as it was the easiest focal length to use with rangefinder cameras like the Leicas and it carried over to SLR film cameras where the f1.4 speed helped when common film emulsions were ASA 400 or slower. Quality autofocus zoom lenses also did not exist, with a few rare exceptions, and so this forced people to use prime lenses.
Completely different situation today with f2.8 zooms and ISO 6400 DSLR cameras but there is still a belief that a serious photographer will only use prime lenses on the part of non-pro photographers. On the other hand a lens like the 70-200mm is large and conspicuous and it is going to limit your street photography as a result.
There are multiple elements affected by the focal length of a lens and this includes image magnification, image size as percent of viewfinder/sensor, compression, and field of view. Compression and reduced field of view are two reasons for going with a long telephoto lens and this can make for more flattering pictures of people without requiring much thought on the part of the photographer.
Personally I find that the area in a scene that captures my attention is best expressed or captured with a 105mm lens. It provides the image size I want with double the camera to subject distance of a 50mm lens. People have a personal space that varies by culture, age, gender, and their perception of the photographer. Get too close and people will tense up unconsciously and this shows in the pictures that are taken.
I also like the 105mm focal length on a full frame camera as I can use it both indoors and outside. Longer focal lengths are difficult to work with indoors and this includes indoor markets where I take pictures of vendors and shoppers. I don't have much working space and a longer lens would be much more restrictive.
Overall the lens I use for 85% of my people photographs is a 24-70mm f2.8 lens that provides the field of view options I want and is fast enough to use indoors and at night with more than enough light for the autofocus sensors to respond quickly and get shots in the moment(s). The 24-105mm focal length range is even better but I have had problems in the past with low light autofocus with the f4 maximum aperture. With current cameras it might not be a problem and if I was a Canon shooter I would probably own one.
