Why did dinosaurs and other similar creatures become extinct?

They think it was a big Asteroid that came slamming into the planet and knocked em out, and those that survived, their decedents had to adapted and are what we call - birds. But but there were already plenty of dinosaurs about, little and big that were what we'd call 'bird like'. So what we call birds are essentially dinosaurs in the same way that we're apes. I mean just look at a dinosaur fossil foot and a Emu or Cassowary foot and they're one in the same.