Here's why:

* Fusion Process: Nuclear fusion involves the merging of atomic nuclei. The most common type of fusion reaction that's relevant to stars and future energy production involves the fusion of two isotopes of hydrogen: deuterium (²H) and tritium (³H).

* Fusion Reaction: The fusion of deuterium and tritium produces a helium nucleus (⁴He), a neutron (n), and a large amount of energy.

* Helium as the Product: The helium nucleus is the primary product of the fusion reaction.

While helium is the main gas produced, other products can be formed in different fusion reactions, but helium is the most prevalent.