There are 2 reasons for this; the first one is that antibiotics are inactivated at high temperatures, so they are usually filter-sterilized and added to molten autoclaved agar once the temperature is 45-50C. The other reason is to avoid excessive condensation forming in the medium. If you pour agar that is too hot, condensation forms on the lid of the Petri dish, which then drips onto the surface of the agar. If you are drying your plates with the lid ajar. if you use the agar plates immediately, the condensation on the agar surface will cause smearing of bacterial growth, and thus no isolated colonies.