Winner takes all.
First of all, the emergence of life is an extremely rare event and
probability for it to happen twice on a single planet (even the whole
universe) is highly unlikely. Life starts with self replicating
macromolecules (current prevailing hypothesis). Once a macromolecule
acquires this revolutionary capability, it will soon overwhelm all rivals.
Like extremely efficient computer viruses, one winner triumphs all, taking
all the resources and spaces, leaving no room for competitors.
An analogy is the big bang, probably it only happens once in our universe.