I suppose it's a manpower issue. I guess showers would only happen during the daytime, and outside of meal-times and supervised yard-time. I suppose they could get 3 prisoners per hour through the shower, and probably 40 hours per week shuffling them back and forth to showers. So a pair of guards would be fully occupied doing nothing but supervising showers for 120 inmates in solitary. These numbers my be wildly off, but not an order of magnitude off, and the manpower issue still stands.