Most District Attorneys were hired out of Law School by whoever is the District Attorney in their home county at the time they graduate from law school. These assistant DAs work for 20-30 years till the DA who hires them retires and they run for his office. During the 20-30 years the assistant DA go to the various County BAR functions, Police functions etc and build up a network of friends and potential supporters among Judges, Fellow Attorneys, Law Enforcement officials and keep they name in the press by convicting criminals.
Most Assistant District Attorneys leave the DA's office long before the present DA decides to retire. To a degree the present DA "selects" his successor by giving him or her the better cases, cases that are not only winnable but good sound pieces in the local media. Thus among lawyers, we often know who will succeed who years before the present DA retires. This is also true of the assistant DAs in the office, they know who is on top and end up supporting that person for they like to keep their present jobs.
Given the above and the tendency of people hiring people like themselves, gives white males several legs up against any other group For example in my home county the last DA to "Retire" was elected a Judge and then his assistant, a White Woman, succeeded him. The only difference from previous successions was his assistant had been female instead of male. In fact all of the DAs in my county going bsck decades were later elected Judge and succeeded by their chief assistant as DA (All except for the present DA, were white males).
Now, support from the law enforcement community is important in electing a District Attorney (DA). Law Enforcement tends to be the domain of white males and thus tends to prefer white males. On the other hand, they also tend to go with someone they have been working with for years, and that tends to be the chief assistant of the present District Attorney (DA).
Also remember, each county has their own District Attorney (DA). Thus in my county which have Zero practicing non white attorneys, will not see a non-white DA for many more decades (and the non-white attorneys who do begin practice in my county tends to be hired by a large utility or other large company and we never see them again). This is true of most of the Counties around my home county, low number or no non white attorneys thus no non white assistant District Attorneys. That is NOT true of large urban areas but such large urban areas rarely exceed one county (Your top ten cities in the US do exceed one county, but not your "smaller" large cities, thus New York City, Chicago, Boston, Las Angles, San Francisco etc AND THEIR SUBURBS cover several counties, but smaller Cities, like Pittsburgh, Cleveland, Buffalo, tend to cover one counties, with only very minimal extension into neighboring counties.
In those smaller Cities, you may have several non-white assistant District Attorneys (DA), while all of the surrounding counties having none because most if not all of the non-white attorneys set up their practice in those smaller cities and only go to the surrounding counties if they have a case in that county. In my area that leave ONE county with a pool of non-white assistant District Attorneys to even think of running for District Attorney, and over 20 counties where the pool of potential District Attorneys (DAs) limited to white males and females (i.e. 20 counties with no non white attorneys in the local BAR).
This is the biggest problem, you have counties with several full time assistant District Attorneys to handle the case load, and more counties that hire part time attorneys for that role do to the population of the county not being large enough to justify full time assistant District Attorneys. These smaller counties tend to be almost all white.
Now, in the South you have many non-white rural counties and it is in these counties you see African American District Attorneys more then in Urban areas but it shows what I am discussing for the rest of the nation, i.e. what is the pool of not only people by lawyers and African American Attorneys tend to go to African American areas where they came from OR reach into Corporate America. The issue is the POOL of potential DAs and in much of the US that is restricted to white males. This is NOT an intentional restrictions, many of these rural BARs make an effort to make such non white lawyers comfortable, but unless they have family connections with the rural county, they do not stay, thus "preserving" the pool of white males to be District Attorney (DA).
Thus the reason for mostly white District Attorneys (DAs), is that most Counties have large white rural populations. Where there is a large non-white populations, Urban Areas and rural Hispanic, and rural African American areas, you get non-white District Attorneys (DAs). Thus it is more a product of who lives in a county then anything else and in most counties the population is overwhelmingly white.
For example Washington County Mississippi has a African American DA:
http://www.washingtoncountyms.us/district_attorney.html
Washington County Mississippi has a 71.3% African American Population for its 51,137 population:
https://en.wikipedia.org/wiki/Washington_County,_Mississippi
To Washington's North you have Bolivar County Mississippi which has an African American FEMALE DA:
http://www.co.bolivar.ms.us/#!district-attorney/cze4
Bolivar County Mississippi has a 64.5% were Black or African American, for its population of 34,145.
Both of the above Counties are along the Mississippi River and those counties tend to have overwhelming African American populations. This is true of most of those counties along the lower Mississippi River (These counties also went for Obama, as the rest of Mississippi voted GOP).
These DAs have family in those counties, they have lived in those counties since the days of slavery. This is much like Rural White populated counties, they vote for someone who is local and in most counties that is a white male.
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to local results. How many black prosecutors would anyone expect in Idaho or Montana? Why should anyone be surprised if it's zero? Black population in those states is practically non-existent, under 1%. A number of other states they called out still have less than 5% black population, often much less (Colorado, Maine, Nebraska, Oregon, South Dakota, Vermont, Wyoming).
