If I understand your question well, you want to know why the L-configuration of alanine is occurred in proteins. L-alanine is occurred in almost all protein, e.g. worm-silk contains 25 %. Cystein, serin, etc., are substituted L-alanine derivs. In the nature, not only the alanine, but other amino-acids have exclusively L-configuration because of steric reasons. Stereoselecitivity (chiral recognition) of the sites where the amino-acids are formed cause it. When the starting compounds are coordinated, the direction of the coordination is controlled (re or is) by steric factors, thus the configuration of the product cannot be only one among them two possible.

Truly said, initial L-configuration preference of amino acids were most likely set by chance. (In pre-biotic environment, chance of formation D- or L-amino acids were equal, but at certain level of biogenesis, some L-aminoacids using forms become better fit to the environment so preference started.)

Thanks for your explaination