Why? Because his 9-9-9 tax plan raises the average American's tax burden by thousands of dollars. And its simplicity surely cuts both ways here: it surely isn't too hard for many families to figure out that they'll be paying more -- much, much more -- than they would under the current tax regime.
Even if you ignore the incredible inequity created by regressive flat tax plans, they're always going to be politically hobbled by the tremendous burden they put on lower income people -- people who make a larger portion of the voting base than the economic base.
And isn't it funny how libertarians who so dearly believe that every aspect of life is governed by microeconomic principles also so quickly forget that the marginal utility of additional dollars decreases like the marginal utility of virtually everything else?
*There may be other reasons Herman Cain is unelectable. E.g., he's a doofus who hasn't spent twelve seconds thinking about policy (or, seemingly, anything else).