Several good answers given to this question but I don't think "black people don't go to therapy" answers the question "why are there so few black psychologist?". You don't really need black people to go to therapy for there to be plenty of black psychologists. But the reasoning behind this is the reality that black psychologists would mostly only be treating black patients, and since there aren't enough black people seeking help, there just aren't many jobs for many qualified black psychologists to hold.
The subtle implication in all this is that white people won't go to see a black psychologist. If they were willing to see one, black people not going to therapy would have no bearing on the amount of practising psychologists that are black. And the same applies to many different industries, especially in the financial industry were most customers who are white don't want to entrust their money to a black person. But that's all fine, that's not really the problem, most races prefer to do business within their own kind, especially when it relates to personal/health/financial reasons. The problem is that unfortunately black people don't give preferential treatment to go to black owned businesses, I always smh when i see black friends go to a white accountant, or white owned dry cleaner, restaurant or any service/goods that could be bought from a black person that is also just as accessible. just why??? these people wouldn't go to you if you owned their business, so why would you go to theirs?? And that is why AA are still mentally enslaved people, they go no love for themselves.