... or, keeping it more simple, perhaps its because on a turbine the starter doesn't need much torque but has to turn the engine at around half its max speed, therefore the speed range it has to work over in starting mode and generating mode are not that far apart (1:2 or so) whereas a piston starter has to have lots of torque but only needs to turn at about 10% of max speed - ie a 1:10 speed ratio.
So its all about the required gearing!
HC