Why is this the case? Because the less surplus income a peson has, the less they are able to afford to pay taxes. But they key is to avoid brackets and to keep the rate increases moderate at all levels, so as not to punish success. Rebates and deductions are pretty ideal in this regard.
The big downside with letting the poor off the hook for taxes, however, is that they benefit from the taxes paid by others, which simply encourages them to want more entitlements. Sometimes, I think it is too bad that we've got an obsession with letting everyone vote, even if they don't understand anything about government, contribute nothing to its operation, and can't find their own country on a globe.