Well, that's the question isn't it? Is it just a frozen accident, or is there some nonarbitrary reason for the left-handed molecules to be favoured?

Sure but that might be an unknowable problem. What if the difference in likelihood were 60/40.

You could go down all sorts of rabbit holes and none of them would truly be falsifiable unless you observed an enantiomeric lifeform on some distant planet.

Perhaps aliens eat right handed life, but left handed life is poison to them.

Seriously. It would be a pretty good selector, and said “alien” need be no more than a snippet of RNA - and it would be entirely gone from earth now, eliminated by us sinister life forms.

The only evidence would be the ubiquitous absence of R-entantiomers in life.

I think I might be lifting from Asimov - The Left Hand of the Electron.