Thanks for the correction :)

Cold air is a lot denser, as the particles that make it up move less. So, hot air gets pushed upwards, as it moves more. Think of it as a person trying to move through a crowd.

Hotness is kinetic energy that's been transferred as heat. If you move a lot, you get hot and sweat, but you stay still, you cool down. There's no active movement in the upper atmosphere, so naturally, it gets cold.