It have higher vapor pressure than NaI and is cheaper and available.

If you look carefully, chlorides are sometimes used as a part of metal halide chemistry. AFAIK, chlorine atoms do not work as well for quartz wall protection as iodine do, and are more aggressive to tungsten electrodes.

The cost of a starting material has absolutely nothing to do with final filling mix cost in MH tech. Probably with the exception of rare earth salts. It has to be highly purified and anhydrous. With preparation costs added, final cost of NaCl, NaBr or NaJ for lamp filling is expected to be approximately the same (and much higher than kitchen-grade salt).

@RRK: The problem is that NaI have much lower vapor pressure than metallic sodium and I think also NaCl, resulting in my BLV Colorlite HIT-DE 70W "Orange" MH lamp take longer run-up and hot restrike time than my other colored MH lamp, and also: The sodium line in my said lamp, have the same intensity as the mercury lines, resulting in a color more like 530 halophosphate fluorescent lamp rather than orange.

It is impossible to use elemental sodium in quartz MH lamps. Without free-iodine or bromine rich cloud near the walls, sodium atoms will attack quartz, and the lamp will blacken just in a few hours. That's what the whole metal-HALIDE deal is about. Interstingly, CMH bulb made of alumina is probably resistant to alkaline metal vapors even without halogens added, but for some reasons pure metal fill is not still practiced AFAIK.

 

With most metals, halides have higher vapor pressure than the metallic elements themselves, but with sodium it is just the opposite.

And if you have highly ionizing environment (which the arc is), it does n ot matter which element which halogen came with, they will rearrange into the lowest energy combination anyway. So an idea "use metallic Na and the other elements in halides" won't work, the Na will get combined with the halogens as well, even as the first element in many cases, so effectively steal the halogens from the other components so leave them as bare metals.

With CMH vs halides it is even more interesting: The alumina quite readily dissolves into any practically halogen salts (it is in fact the way how aluminum is made - by dissolving the Al2O3 into a salt and extract the Al by electrolysis of that mixture), so the CMH life limit uses to be the time how long it take for the liquid halides pool to dissolve a hole through the burner wall.
And related to that are the many design tricks intended to prevent the liquid halide pool to be touching the part of the burner that needs to be thin (to pass the light) during operation...