I’m not sure if anyone has mentioned it but aside from the rate of refueling and actual ship speed being slower, there is also the problem of smoke; coal fired ships show an enormous plume that is easily visible to U-boats and aircraft from a great distance, oil fired ships do not. A convoy of coal fired liberty ships would be moving slower and visible from over the horizon.
Coal fired ships can be low or no smoke. It requires good firing, high quality coal and well maintained fire boxes and flues. Oil fired ships can be just as smoky as coal burners. Again good firing, good quality oil and well designed and maintained burners. And in the 6-12 knot range it wouldn't make a difference if coal or oil were used.
The two big differences are the labor and skill needed to operate a coal burner and the logistics of transporting the fuel needed all around the world. Oil is much easier to transport and distribute than coal is, especially to remote non fixed base locations.