Why can't emu's fly

i just wanna know why they can't fly cause all other birds can fly so why can't the emu

Mother nature's policy on muscle is, "Use it or lose it."

Historically the Emu had plenty of food and few natural enemies, so there was little reason to fly. It evolved into a creature with a bigger, heavier body but without the strong wing muscles needed to pull the weight load into the air.