If you aren't a lawyer, you can't be a DA.  You can't be a lawyer without passing the Bar Exam.  

There is no 'conspiracy'!  

 It's just too expensive to run for an office no one really cares about that the incumbent usually wins.

Most attorneys make more money in private practice than either the DA's office or as a  Judge.  A lot of them start in the DA's office for the trial experience and then join law firms.   

Most Judges are appointed by a Governor most often because of political favors.