A free neutron decays into a proton, but a free proton does not...

For a spontaneous decay process to occur, the rest mass of the initial particle must be greater than the sum of the rest masses of the product particles to satisfy the principle of conservation of energy.

The rest mass of a neutron is slightly greater than the rest mass of a proton ($m_n = 1.00866$ u and $m_p = 1.00727$ u).

Thus, the decay of a free neutron into a proton, an electron, and an antineutrino ($n \rightarrow p + e^{-} + \bar{\nu}_e$) releases energy and is energetically possible.

Conversely, a free proton cannot decay into a neutron because it has a smaller rest mass, and such a process would require an input of external energy.

Therefore, a free neutron decays into a proton, but a free proton does not decay into a neutron because a neutron has a larger rest mass than a proton.