Emus are birds, so why can't they fly?

"Though the common ancestor of all modern birds could fly, many different bird species have independently lost their flight. Flight can have incredible benefits, especially for escaping predators, hunting and traveling long distances. But it also has high costs: consuming huge amounts of energy and limiting body size and weight."

Here's a TED video talk on why some birds can't fly:

https://thekidshouldseethis.com/post/flightless-birds