FICO score is used to measure risk of default, and hence it deals with probabilty and percentages. As long as the score range statistics aren't too volatile, then the lenders can calculate with a good degree of accuracy how much to charge in fees, penalities and interest. Hence the oftime predatory lending practices of the sub-prime market.
I would not like to have my income as a matter of quasi public record that could be sold to marketeers, so I'm glad that income isn't factored in..