Because the flat tax is paid on all income above that threshold, the rate can be very low. It ranges between 13% and 33%. The low rate encourages payment. There are no tax loopholes, nor the need for them, given the low rate. Instead of paying accountants to shelter income and move it offshore, people find it cheaper just to pay the tax. And a low rate makes it more worthwhile to earn more, which brings economic expansion.
A flat tax is simple, letting people understand their obligations. It is fairer, with low earners paying nothing and the rich paying their due. And it unleashes all the talent and enterprise being held back by a devious and complex system.