The truth about the asteroid

Because it blew up in an explosion big enough to wipe out all other life on the planet.

^This is a smart person. Everyone else learn from richiro's example.

Maybe the UFO didnt outright crash into earth but just hit earth gravitational pull causing the tides to rise thus drowning the dinosaurs and making fossils out of them. The ship we came on had enough back up power to sustain itself but not enough to pull away from earth gravitational pull ( or the boosters were damaged) so the aliens stayed it the UFO until there was no possible way to survive in the ship. Then the remaining few who survived the deadly crash landing onto this planet looked up into the sky passing down the name of the ship for generations. The Moon.