Answer
Only L-amino acids are used because a mix of the L and D amino acids would result in a different protein.
Work Step by Step
A macromolecule requires very specific amino acids in specific positions to be formed. Any change in an amino acid or in it's position would result in a different protein being synthesized. So if a random mixture of D and L forms was used in the process, the amino acid sequence would specify many structures as opposed to just one.
