What factor causes the overwhelming majority of stars in the Milky Way to be cool relative to our Sun?

Answer

The extreme prevalence of M-dwarfs is tied to their slow rate of fuel consumption.

The long-term accumulation of slow-burning stars, like M-dwarfs, whose longevity far exceeds that of massive stars, results in them dominating the current stellar census.

What factor causes the overwhelming majority of stars in the Milky Way to be cool relative to our Sun?
astronomytemperaturestarsstellar evolutioncommonality