Likely we are all left handed on earth because our left handed ancestor outcompeted the right handed organisms in the primordial soup. Or the right handed organisms just didn't evolve in the first place here on earth and there was nothing to outcompete. There might still be some higher order advantages to shifting chirality one way or another. Certain molecules, such as methamphetamine, have differing bioactivity based on chirality. Maybe this can be regulated in some way such as to control the rate of some other downstream process. In an abstracted sense, chemists here on earth are already this organism as they refine reactions to produce desired chirality and reduce expenditure on undesired chirality.
ET could be using different amino acids, or more or fewer. I would hazard to guess there is immense selection to reduce the amino acid set to its most necessary components. This pressure has gotten to the point here on earth where even these necessary components might not all be produced endogenously by the organism who needs them, but consumed from the environment saving energy spent on synthesis. But this requires your neighbor to be producing these AAs, such that you consume them, and you having sufficient feedback mechanisms to not immediately consume all of your neighbor's species and put your own insufficient lineage to extinction.