Because the lowest 50% (in income) of taxpayers don't pay income taxes. Reducing income taxes won't benefit them at all.
For the lowest 50%, the payroll taxes (Social Security and Medicare) are the only federal taxes they pay, aside from federal excise taxes.
Now that you understand the distinction, you can easily figure out why Obama and the rest of Democrats want to reduce the payroll tax "temporarily".
But, since Social Security is already paying more benefits than it receives in payroll taxes, this makes the situation worse. So, more money must be drawn from the "general fund", which is funded by income taxes.
See above for who pays income taxes. So, reducing the payroll tax is nothing more than another form of "soak the rich".
Yes jl, this was the point I want to make.
Switching from promoting a “payroll tax” cut to an income tax cut would bring this home to more people one would hope. Hell it may even work.