Mostly: because billions of years have passed. Even if it turns out there we millions of different self-replication mechanisms that were complex enough to give

Mostly: because billions of years have passed. Even if it turns out there we millions of different self-replication mechanisms that were complex enough to give rise to (an abstract form of) competition, early Earth has a period of hundreds of millions of years in which each and everyone one of those had plenty of time to run their course to extinction. Remember that that single common ancestor was _not_ alone on the planet. They might have been millions or even hundreds of millions of other potential ancestors, but if there were, all of them died out before the start of the fossil record.