Here's why:

* The Early Universe: The early universe was primarily composed of hydrogen and helium. These elements were the only ones created in the Big Bang.

* Gravity's Role: Over time, these elements clumped together due to gravity, forming massive, dense clouds.

* Nuclear Fusion: Within these clouds, the immense pressure and temperature eventually ignited nuclear fusion, the process that powers stars.

While other elements exist in the universe today, they were formed later, through processes like stellar nucleosynthesis (the creation of heavier elements within stars).