Here's why:

* Abundance: Hydrogen is the most abundant element in the universe, making up roughly 75% of its mass.

* Emission: Emission nebulae are clouds of gas that are heated by nearby stars. This heating causes the hydrogen atoms to become excited and emit light, primarily in the red part of the spectrum.

* Characteristic Red Color: The iconic red color of many emission nebulae, like the Orion Nebula, is a direct result of the emission of hydrogen.

While other elements like helium and oxygen are also present in emission nebulae, hydrogen is the dominant gas by far.