Other reasons that may have contributed for that preference, is that left-handed AA may be required for catalytic production of other left-handed organic molecules (sugar, lipids). Although this is kind of a chicken and egg thing. More surprisingly, some bacteria actually use right handed amino-acids: "From the ratios of right- to left-handed amino acids in seawater, McCarthy and his colleagues conclude that a substantial fraction of the dissolved organic matter comes from bacteria. This challenges the traditional view that algae produce most of the ocean's soluble biological material. Bada notes that bacteria coat themselves with right-handed amino acids because the unusual structures provide a tough exterior that resists other organisms. This is what helps bacteria evade digestive enzymes in human stomachs, he says."
Interestingly, a new hypothesis suggests that while reactions may produce equal amounts of L and D, the L AA dissolve better in water..."Donna Blackmond at Imperial College London and colleagues dissolved a mixture of solid
So should we expect life on Mars to also be left-handed if it exists?