Progressive taxes were designed so that people provided support for the nation in the amount that they were able to do so without undue burden. Unfortunately there are already too many tax shelters for the rich to hide their money and continue accumulating wealth without helping society, already putting too much burden on the working class and poor. A flat tax would increase this burden even more, causing many families unnecessary financial hardship and encouraging crimes of desperation.
As somebody who grew up in the upper class; the old adage of "the rich get richer and the poor get poorer" has never been more true. People with money to spare, pay barely any tax; while people who struggle end up losing a huge chunk of their paycheck to taxes. The rich should be taxed proportionately on their wealth; so that the less fortunate can have the room to climb up the socio-economic ladder.