Why is Hydrogen the most abundant element?

The reason hydrogen is the most abundant element in the universe goes back to the Big Bang. 

The Big Bang quickly led to the formation of protons, neutrons, and electrons. Because hydrogen is the simplest element, it formed most readily. 

Technically, even a lone proton classifies as an atom of hydrogen. A neutral atom also has an electron. Most hydrogen atoms don’t have any neutrons, although the less-common isotope deuterium has one neutron and the rarer isotope tritium has two neutrons.