So say I begin with a lump of pure, solid ammonium hydroxide, NH4+OH-. As I heat it past its melting point, it undergoes a phase transition and becomes a liquid. At the same time that happens, it is also ceasing to be NH4+OH-. It is becoming a solution of NH3 in H2O. As the solution heats up, the NH3 boils away, which shifts the equilibrium to create more ammonia, until there is no more NH4+ or OH- left in solution, and all you are left with is pure water.
Whereas with Na+OH-, the initial conversion to H2O does not happen, as it does in the case of ammonium hydroxide, as the sodium is not protic and cannot donate an H+. So as you heat it past its melting point, all you have is a puddle of ions... which are involatile.
Is that the long and short of it?