Why do most stars lie along the main sequence on the Hertzsprung-Russell diagram?

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Open cluster NGC 290 — Star Stars in the open cluster NGC 290, as seen by the Hubble Space Telescope.

Most stars lie along the main sequence on the Hertzsprung-Russell diagram because this phase represents the longest and most stable period of a star's active life.12

During this time, a star is powered by the nuclear fusion of hydrogen into helium within its core. Because stars spend the vast majority of their existence in this hydrogen-burning state, they are statistically most likely to be observed in this region of the diagram.132

The specific position a star occupies on the main sequence is determined primarily by its mass, which dictates its luminosity and temperature. Once a star exhausts the hydrogen fuel in its core, its internal structure changes rapidly, causing it to move away from the main sequence and evolve into a giant or supergiant.132