Here's why:

* Nuclear Fusion: Main sequence stars are powered by nuclear fusion reactions occurring in their cores.

* Hydrogen Fusion: The primary fusion reaction in main sequence stars is the conversion of hydrogen nuclei (protons) into helium nuclei. This process releases immense energy, which is what makes stars shine.

* Proton-Proton Chain: The most common fusion reaction in stars like our Sun is called the proton-proton chain. This chain involves a series of steps where four protons are ultimately converted into a helium nucleus, releasing energy.

While other fusion reactions involving heavier elements occur in stars as they evolve past the main sequence, hydrogen fusion is the dominant process during this initial stage.