The original AAA county committees in the 1930's were specialized by crop and function. They were a means to tap local knowledge of yields and production histories. They fit into the general left perspective of pushing local democracy. They also had the advantage of involved local community leaders in the program, securing their support for it, and in many cases for the Democrats who created the AAA. (Roughly speaking, the committees are elected by farmers in the county.)
Over the years the role of the committees has changed, their functions have diminished, the supervision from Washington has increased. People, both auditors and others, pay more and more attention to how the bureaucracy operates and whether there's consistency from place to place. And local option as represented in the committees makes consistency very hard to achieve. Technology makes it much easier for Washington to provide detailed direction, reducing the autonomy of the committees. Perhaps the biggest force in these changes has been the civil rights revolution. That seems to start in the 50's and 60's, but in fact a couple years after the AAA of 1933 was passed there were protests over the (mis)treatment of sharecroppers by some of the committees.
The latest step in this evolution is Sec. Vilsack's announcement that he'll appoint voting members to committees who are representatives of "disadvantaged communities". He was given this authority in the 2002 farm legislation, or rather Sec. Venneman was, and there's no explanation for the 10-year delay in using it. (There had long been provision for non-voting members on the committees.)
It's sort of ironic in the broad view: to the extent the civil rights movement worked to reduce the power of the committees, they have reduced the potential gains from having voting representatives
on the committees.