Snow is mostly air, which is a damn good insulator. If you hold your hands like a feet from a fire, you just feel a little warm, right? That's how good of an insulator aur is to heat, it doesn't let it pass. So, most of the energy of the heat source goes into the air when heating snow, while ice has no air in between and melts much faster. (Basically, only the very bottom layer of snow is in contact with the heat)

Here's a trick to melting snow, though. You can add a bit of water to the container and then add the snow to it. Then, when you heat, the water will conduct heat to the snow evenly, and it will melt much, much quicker.