Chirality and symmetry breaking
There's no mystery at the chemical level. L-amino acids form polymers (proteins) with other L-amino acids, and D-amino acids with other D-amino acids. These chains then fold up into spirals and sheets. Spontaneous bonding between L- and D- amino-acids is chemically unfavorable, because the molecules don't fit together properly.
So, if the first self-replicator was L-based, that would have fixed life (or pre-life) on the L-form, and soon the D-acids in the environment were reprocessed by it/them into small non-chiral molecules (i.e. used as food). Once the L-basis of life was established, it could never change. L-based life can't assemble things out of D-bases, but it can and does use them as fuel when they arise spontaneously.
The deep question is why was the first self-replicator to use amino-acid polymers based on the L-form? The answer may be that symmetry was broken at random. The pencil balanced on its point had to fall one way or the other. If so, and if we can ever find any other life to study, there's a 50% chance it'll be based on D-amino acids.
However, there's a much deeper broken symmetry that is itself chiral. The weak nuclear force. Because of this, the binding energy of L-amino acids is very slightly greater than that of D-amino acids. In a mixture formed by inorganic chemistry from achiral precursor molecules, there will be about 100 more L-molecules per mole than D (i.e. 1 part in 6.10^21). Was this enough to tip the balance? Is L-based life universal, thanks to a physical symmetry that broke almost immediately after the big bang itself? Can we ever know?
Back to trivia. In a few cases some living organisms manufacture an L- molecule and others the same molecule in D-form. What's the difference between lemon flavour and lime flavour? One is L-, the other is D-limonene! Why is 7-up "Limon" flavoured? Because in a chemistry lab, it's far easier to cook up a racemic (50/50) mixture, and that's what your fizzy water is flavoured with.