From what I know, SBMV lamps are only slightly more efficient than incandescent. Why is this?

Any lamp with an incandescent ballast will be considerably less efficient than a standard magnetic ballast. There are probably other reasons but heat lost from the filament is one. Same is true for all those F14 fixtures using a ballast lamp.

First and most important is what Sol said

Second : With Magnetic ballast, the phasor voltages across the arc and the ballast are at near right angle to each other, which allows fairly high arc voltages - and efficient arctubes. With Resistive ballast, the phasor voltages are near inline with each other, so the available voltage for the arc is much lower - And the arctube less efficient

On 120V its even more extreme : With Magnetic ballast, the ballast is an autotransformer that steps up the voltage, so allows making high voltage efficient arctube (arc tube alone is over 120V). With Resistive ballast, there is no step up available so its just 120V, and that have yet to be split to arctube and filament - There is barely voltage headroom to even include an arctube in the circuit at all



Phasor voltages - fast explanation (using 240V mains and simple choke as example) :

Voltage across the lamp + Voltage across the ballast = 240V

In DC, this would be as simple as addition/subtraction. In the case of resistive loads (which is sorta the case with the MBFT) that holds on AC too. That means, if we want say 150V on the ballast for stability in case of line voltage changes, the arc can be 240V - 150V = 90V max

In AC and loads like a choke, the voltages sum up like vectors on a plane, not as numbers. So you can get for example, stuff like 125V on arctube + 200V on ballast = 240V. In this example, there are whole 125V across the arctube - so fairly efficient arctube, and at the same time 200V - well over 150V on the ballast coil. And both are only 240V together, not 325V as would b eon DC

If you want better explanation/drawing ask away