Here's why:

* The Big Bang: The universe began with an incredibly hot and dense state.

* Cooling and Expansion: As the universe expanded, it cooled. This cooling allowed particles to form.

* First Elements: The first elements to form were hydrogen (H) and helium (He). These were produced through a process called Big Bang nucleosynthesis.

* Dominant Element: Hydrogen, being the simplest element, was the most abundant element created. It makes up about 75% of the mass of the universe today.

So, while helium was also produced early on, hydrogen formed first and remains the most abundant element in the universe.