[adrian_b]

The lamps that use alkaline metal vapor instead of a noble gas have better energy efficiency, because of a lower ionization potential, which leads to a lower voltage drop on the lamp. Therefore they have been preferred for lighting instead of neon lamps and the like.

Among alkaline metals, sodium is the cheapest, so it was a logical choice.

However, the fact that it produces light of a suitable color was a happy coincidence. If sodium had produced violet light, like potassium, and potassium had produced yellow light, potassium would have been chosen for lamps.

So among the criteria for choosing sodium for lamps, the color of the light was as important as cost, ionization potential and vapor pressure.