Why are the elite called the 'upper crust?'

The lions or crack men of the day. The phrase was first used in Sam Slick. The upper crust was at one time the part of the loaf placed before the most honoured guests.

Further to Dark Bark's answer:

Bread was put, as a raw lump of dough, straight into the bread oven. No bread tin, it just sat on the floor of the oven. The oven was heated by the fire and was very hot at the bottom. When the bread was taken out to cool, the base of the loaf was overcooked, black and dirty. The top of the loaf was just right, and still clean. The bottom of the loaf was given to the servants to eat, while the upper crust was for the master of the house.

http://www.rootsweb.com/~genepool/sayings.htm

Yes, In the Middle Ages, bread was dispensed, even at formal meals, by diners tearing off chunks from a big loaf. Heaven forbid that the bread of an aristocrat might be touched by the hands of a commoner! So it became the custom to slice off the upper crust of the loaf and present it to royalty (or whoever was the most distinguished person at the table), both as a way of honoring the elite and of keeping out the potential germs of the riff-raff...