Because most black officers from around the St. Louis area don't want to work in North St. Louis/Ferguson.
It's too dangerous.
I wonder what the pay is for a Ferguson police officer?
Keeping in mind its a small town with normal budget restraints, and that 2/3 of the population is black, and black officers are on the priority affirmative action hiring list for better paying jobs elsewhere, and a whole host of other valid concerns...
Its a wonder there are ANY police officers willing to work in that hell-hole.