Why is hydrogen the most common element in the universe?

Because it is the simplest form. When the universe originally formed, it was too hot for matter as we know it to exist - there was only energy, quarks, and possibly other particles we have never encountered. As the universe cooled, the quarks began to stick together. And when they did, they formed protons, the simplest form of matter. Which, when combined with an electron, makes hydrogen. While it was still hot, some of that hydrogen fused to helium, but most of it remained as hydrogen.

The universe today is observed to contain one helium atom for every ten or eleven atoms of hydrogen