Why would a tax on what you earn be ideal as opposed to a tax on what you purchase?
Personally, I support the sales tax over the income because it taxes everyone. Right now, there are quite a few people who pay no income tax, and so they dont understand the burden taxes put on people. To me, the only way to treat people the same would be to tax the sale of each item the same.
The problem is that a sales tax taxes everyone. Food, housing and energy costs fall as a percent of income as income rises. At the lower end they take up nearly all of a persons income.
How do you make it politically palatable to gobble up the poor's already scarce resources? Better a Flat tax with a declining rate over time as the economy grows government as a percent of said economy declines.
You then support the poor with a Negative Income Tax.
Look up the Winter 2011 edition of City Journal. They have a great article on it.