Because the rudder on the B-52 is too small to maintain lateral directional control with an inoperative outboard engine in a 4-engine configuration at takeoff power/airspeed. Go google for some pictures, on a B-52 it is less than 10% of the span of the vertical stabilizer, compared to more than 25% on a 747.

And if the rudder were made large enough to maintain directional control, the loads in the aft fuselage would be large enough that the aft fuselage would need to be re-designed.