>Ingredients for baked goods are often divided into two categories:
>structure producers and tenderizers. Shortenings inhibit the proteins that
>form gluten from joining into long, tough networks. That is why they are
>called "shortenings." The fat makes the baked good more tender, improved
>the "mouth feel" by making it seem moist, influences the way some baked
>goods like cookies spread, influence browning, and carries fat soluble
>flavors and/or contributes its own flavor.

That would explain why shortening ruins pizza crust.