Question

why the temperature remains unchanged

Answer 1

As heat energy is added, more molecules escape, and the temperature remainsrelatively constant. Finally, when all the molecules have escaped the solid and transitioned into the liquid phase, thetemperature can start to rise above the freezing point.

Answer 2

when the heat is supplied to a solid at its melting point then due to the latent heat of fusion the temperature of that solid remains the same it is then not changed anymore.