And the economics of nuclear - enormous fixed costs, minimal fuel costs - encourages utilities to run them near full power. So they'll have relatively little spare generating capacity, regardless of their load following characteristics.
OTOH, a gas turbine generating plant is essentially a turboprop airplane engine, with the propeller replaced by a large generator. 1950's mechanical control technology can adjust the throttle on a ~1-second time scale as the load on the grid changes.
And the economics of gas turbines - quick & cheap to build, expensive to run - encourages utilities to run them (or not) as demand requires. So they'll usually have lot of spare generating capacity, to meet demand surges.
edit: I should note there's a sweetspot, as the neutrons have to have enough speed to overcome the repulsive force in order to enter the nucleus. Thus the cross-section has a maximum at some energy (speed), and it varies a lot between elements and isotopes.