This topic keeps coming up... we'd really need a FAQ here...
Long story short: FD experimented with railguns of higher class many years ago. They found no satisfying way for them. Due to the way the weapon works, they either felt lackluster for a weapon of that size, or was overpowered.
There was just no way to tune them in a satisfactory way. Due to their unique performance i can very much believe that. In the end, ED has bigger fish to fry, especially in its current state.