Plastic coating tends to be very thick and therefore closes the gap in the ends of the coils. The problem is that when the gap is closed, the spring rate can change significantly. Another downside is that plastic adds unnecessary weight.
Chrome plating could be used, but can induce hydrogen embrittlement, which can result in spring failure. Therefore, manufacturers do not generally offer this option.
Epoxy powder can be applied within very fine tolerances. Therefore it is possible to avoid coating build up that would impact the spring rate.
Bottom line is that a serious racer should use epoxy coated springs. Hyperco only manufactures epoxy coated coils.