Because a 1000 ton pressurized water reactor driving steam turbines is just the ticket for interplanetary space travel. There is a reason that SPACEX uses LOX and kerosene.
It may be a bit difficult keeping a nuclear unit cooled without adequate liquids in space, but nuclear “thrust” technology that does not require steam turbines is applicable to space travel within this solar system. Nuclear thrust propulsion theoretically will replace turbines and propellars.