Why there is no neutron in a Hydrogen atom?

Hydrogen exists as three different isotopes. H-1, H-2, and
H-3.  H-1 is hydrogen with a proton in the nucleus and exists in nature as H2. This is
the most common form of hydrogen.

H-2, also known as
deuterium, has one proton and one neutron in its
nucleus.

H-3, also known as tritium, has one proton and two
neutrons in its neucleus. Tritium is a radioactive
isotope.

So while the most common form of hydrogen has no
neutrons in its neucleus, other isotopes do.