Why are sugars found predominantly in D-form in biochemistry?

The basis for homochirality in biology is still an open one. There have been a number of hypotheses proposed, and are being tested and examined as we speak. I would also note that your question - generally speaking - also applies to the homochirality observed in amino acids.

 

L forms of amino acids

Here are some suggested mechanisms for homochirality in amino acids. As Mike H mentioned, it is still a very open topic.

Photochirogenesis:[/PLAIN] [Broken] Photochemical Models on the Origin of
Biomolecular Homochirality

 

Perhaps I am stating the obvious, but biochemical reactions are facilitated by enzymes which act based on the shape of the reactants. If your body has enzymes for D-sugars, L-sugars will be largely excluded from the biochemical pathways.

I recently heard a NASA astrophysicist speak on homochirality. One suggested mechanism is that circularly polarized light (say, from a pulsar) will interact with one chirality and not the other, causing reactions that leave only the other form intact. He noted that amino acids found on meteorites are present in both forms, buts there is a slight abundance in the L-isomer of some. Other researchers present asserted the excess was significant enough to plausibly explain the preponderance of the L-form amino acids in biology.

 

Fewmet,
Can you post a link to the speech?

Aymeric,
Here are some keywords if you feel like digging into the topic:
Murichison Meteorite, circularly polarized light, isovaline,Miller/Urey Experiment