Here's why:
* Main Sequence: This is the longest stage in a star's life. During this phase, the star is fusing hydrogen into helium in its core, generating energy and maintaining its stability. The vast majority of a star's life is spent on the main sequence.
* Other Stages: While stars do eventually evolve beyond the main sequence (red giants, white dwarfs, etc.), these stages are relatively short compared to the main sequence.
Think of the H-R diagram as a map of a star's life. The main sequence is like the highway the star travels on for the majority of its journey.
