|Where the Money Is|
As the accompanying chart demonstrates it's no good looking for additional revenue among low income tax payers. Taxing these folks at a 50% rate would devastate the middle class, lead to the mother of all tax revolts, and still would only raise half a trillion dollars a year.
However, following Willie's maxim and taxing the top 1% at the same 50% rate would raise ~$850 billion a year and leave those wealthy tax payers only slightly less rich. Rates representing penal taxation of lower income individuals barely ruffle the gilded feathers of the rich.
Panderbear previously addressed the fairness issue and the intentions of Founding Fathers regarding the distribution of the tax burden. The Founding Fathers expected and fairness demands higher tax rates for the wealthy than for the less privileged. Add to those the most obvious reason of all. That's where the money is.