Because income taxes are more progressive than payroll taxes. In fact, it's the income tax that's really the only progressive tax we have, and everything else -- payroll taxes, user fees, etc. -- works against it. Even state income taxes are usually only mildly progressive, which, compared to the federal ones, make them regressive.
My slogan: get rid of all taxes except the income tax at the Federal level; and property taxes at the county level.
/jordan