Why are Americans so against the idea of implementing a country wide income tax?

The US is one of the few countries in the world where it separates the responsibility of federal, and state income tax, and the state income tax can be very high according to the state you're in. Is this not a huge disincentive to move states right now, if you have to worry about paying double income tax? If the US simply used a blanket country wide income tax rate, surely it would just be easy for people to move around states and help to disperse the population evenly throughout the country, helping to break up the monopolisation of cities like NY and LA?