If life could arise once, why would the conditions not be amenable to it arising more than once? Sure, life tends to pollute-terraform for itself, but it's a big planet, with more than enough distance between the first occurrence and pristine environment for it to happen several more times.
binaryturtle asked this question above and got about 10 responses. Conditions are very different, precursors to life get quickly consumed by existing life, origin of life is a rare event, etc.