Everyone is equal and has equal votes in government, so therefore everyone should pay the same tax rate. The reason is simple--people who make more money pay more in taxes anyway. To make things fair, everyone should pay the same tax rate no matter what so that the burden is shared equally by everyone in society. Plus, a flat tax keeps it simple and everyone knows exactly what rate they pay.
Progressive taxes are confusing because it is really hard to figure out what your tax will be and why. There are all different things that have to be taken into account when working with a progressive tax. There is income bracket, which can be divided up into all kinds of different ways, and there are deductions for all kinds of things. This is confusing.