Good point, also why isn't there at least one tall hobbit, i mean we have dwarves so hobbits should have 'talls' ?
why weren't there any girls in Lord of the Flies? Why weren't any of the marines in Aliens Gay?
BECAUSE THAT'S THE WAY THEY WERE WRITTEN. There's nothing you or I can do to change it just by bitching about it.
Different parts of Middle Earth appear to be inhabited by different species, rather than different races.
Bear in mind that unless if the Hobbits segregated their black community, any ethnic differences would tend to disappear over the centuries as people of different races interbred.