, i.e. the people in the upper strata
Some theories as to the origin of this expression suggest that it is related to the different 'layers' of bread once it is baked in an oven: The bottom layer was more likely to be burnt and was given to the servant while the upper layer, or 'upper crust', was reserved for the family of the house. I'm not sure that this is a true explanation for this phrase - I've never seen bread cut like that!
If this expression does originate in a bread baking context, it may be related to the fact that the 'upper crust' of the loaf was the the only visible part of the loaf and was therefore the only visual indicator that the loaf was baked properly - i.e. it reflected the quality of the whole loaf.