I think it is because most of those who are recognised as the great street photographers used B&W. All the best street pictures we see are in B&W and so we are conditioned to think that B&W is best for street photography. It's about tradition as much as aesthetics.
While B&W still dominates street photography there are plenty of great examples of colour street photography.
I am about to go on a trip soon. Its too late to learn to shoot RAW. Not going to chance it.
Shall I take in color and later convert some to B&W or
shall I select my shots in either color or B&W for each situation?
But what you can see in those pictures is that each has a limited range of colours... there is always only one dominating colour!
John Goldsmith a photographer who's work I really enjoy, and one particular aspect of his work is his street shots which is in colour (you can see it from his website ) I wanted to bring this up particularly because Eric Kim (another photographer who's work I really enjoy and have attended his exhibitions when he was in town) recently interviewed John Goldsmith regarding his photographic vision and expression, It was quite interesting and informative as he steps away from the typical b&w style of reportage photography and instead captures images from an impressionist point of view.
You can find the Eric Kim's interview from his website here