A flat tax, while it sounds nice, is actually considered a "regressive tax" or a tax that burdens the poor more than the rich because exactly because the poor pay the same percentage of their income in taxes as the rich. The rich can use a higher precentage of their income for non-essential expenses than the poor. If you make $40,000 a year, most of that, like 98% is going towards living expenses, raising the family, etc. If you make $4 Million a year, you might have $1 million dedicated to living, but that is still only 25%. So 5% out hits you less hard than the guy making 40k.
For example:
If you make $30,000 a year and get taxed at a 5% rate you pay $1500. This could mean the difference between heat in the winter and food, depending on your situation.
If you make 300,000 on the other hand...$15,000 is a nice ski trip for the kids to Vail for the week.
The "flat tax" also does not take into account interest, so you billionaires that inherited the family fortune are essentially not taxed at all because they make no actual income.
EDIT: and before anyone jumps all over me, I am just trying to explain why a flat tax is considered regressive. I didn't make it up, and God knows a flat tax would rock for my family.