Question

Why light does not glow when voltage is low

Answer 1

The light usually dims (and at times even doesn't glow) because of low current which in turn is caused by low voltage.

In short it's low voltage.

Current, voltage and power are all linked via the impedance (like resistance) of the thing in question.

The voltage is a property of the grid, with something like a light bulb you can assume the grid voltage won't change. it might if you were running an industrial induction furnace maybe, but not with a light bulb. The current drawn will drop as well, but as a consequence of the voltage drop.

What you are seeing of course is a drop in power P=VI