Diesels are typically quite heavy because of the 17 to 1 or so commpression ratio.Helicopters have favored low weight engines even if thirsty for fuel.
If a long range helo is desired, then a diesel may make some sense, because the fuel weight savings could offset the heavy engine.
My diesel tractor seems to leak almost as much fuel as it burns (just kidding) 5 gallons will run it all day.
Diesels have good low rpm torque and that would help reqain low rotor rpm in flight.
Consider the shaft dynamics and stresses that would be very different for a diesel.