A flat tax rate is a bad idea, because as your wealth increases the incremental utility of money decreases. This is why we have a marginal tax rate system in the US. The highest rates only apply to the portions of income greater than $500k.
The utility of time, however, stays fixed. We've all got something like 75 years to live. A flat tax of 20% takes one-fifth of a working week for the public. And, really, why should 2 days be taken from person X while 1 day is taken from person Y?
Time and money are different things. Many people are not paid on a time-basis. Taking 20% of the earnings of a minimum wage laborer working 80 hours a week is much more burdensome than 20% of the wages of a stock broker who makes $1M/year with their bonuses.
