If they wanted a mass driver that can be built with today's technology, but needed advanced training to repair and maintain, then DARPA would be investing in Coilguns.
However, part of the requirements is that someone with a high school education, and not one that majored in Science, can keep them running.
Mechanically, a Rail Gun is the simplest possible Mass Driver; it consists of two conductive rails, a power feed, and a loading mechanism. A barely literate wrench monkey with a tech manual could keep the thing going.... if it weren't for the fact that current available rail materials degrade with each shot, limiting the usable life of the gun.
A Coilgun would not have the same issues. However, they are far more complex.
Now imagine the nightmare situation where gunners with an IQ in the lower quarter of the possible range have to set parameters for each shot taken with the "smart" AG ammo. Suddenly, making more complex weapons makes less sense when the user may well have an IQ of under 50...
(This is not intended to be an attack on our Fighting Men. Rather, it is an evaluation of how important it is to keep operational difficulty down so the largest number of military men can use the weapon. Plus, the military tends to have the more intelligent and educated recruits be assigned to non-combat roles, where their high intelligence can be better applied, or else have them undergo Officer Training instead. So, the average gunner will tend to be... not significantly above average mentally, shall we say...)