Because it's too much of a lift at once. To naturally get a mirror E-Coli you'd need a separate planet and billions of years and lots of luck.
It's a mostly-incompatible change to all the machinery all the way down.
It's a mostly-incompatible change to all the machinery all the way down.
It's not just about the food/excretions of the hypothetical cell, but about whether its structure and innards are things the rest of the ecology can detect/defend-against/consume.