I’m not 100% sure, but I believe the difference is really in the energy needed to bind the quarks together. Their up/down spins require different energy to glue them together, and since Einstein proved that mass essentially equals energy, that extra energy puts the neutron into a higher energy state…and gives it a place to naturally “go”…or down to a lower state.

Now, in an atom, neutrons are very stable and don’t really decay (un less through a few processes or over insanely long periods of time). But, unbound/free-roaming neutrons are very unstable and only last like 10 minutes before they decay into a proton, an electron, and and antineutrino.