Because Trump's plan is the only one that doesn't balance. The others get the basic math right.
Right for their friends at Goldman Sachs. Take a look at the real costs of Ted Cruz's tax plan for a small business. He routinely says its a 10% flat tax, but its not for a small business owner, or the self employed. For them its a 26% flat tax.
So under Sen. Cruz's plan the corporate executive earning $2.0 million per year pays 10% of his income, but the guy mowing lawns pays 26%.
I'm sure that will get lots of voters in the general election. After all, doesn't every plumber, carpenter, electrician, mechanic and other tradesman want to pay a higher tax rate than Fortune 500 corporate executives?