Earlier quoted context omitted.
Most Republican "flat tax" ideas have tax simplification as a major selling point (e.g. Ted Cruz talking about filing your taxes on a postcard); if you can simplify the implementation of the existing tax schedule, you lose that selling point.
The problem is the existing tax schedule isn’t so clear. Depreciation of assets for example. By “simplifying” tax filing without simplifying the underlying calculations, there is far less public visibility of just how broken the tax system is. A flat tax is closer to good than our current system. People that oppose flat taxes do so because they want other people to pay more. It’s that simple. See what Milton Friedman…
That's a decently idealistic viewpoint, but "complicated" doesn't necessarily mean "broken". For example, pretty much all Unix-style operating systems have not only a ton of now-unnecessary code to handle text I/O from 1960's teletype machines, but they even come with a "terminal emulator" that literally emulates the behavior of a 1960's teletype machine. That's how the command line works. You would never design it that way from the start, it's complicated, it's crufty, and it's vaguely horrifying, and yet it works and solves virtually all of the real problems that real people would have with that system. Just like letting the IRS fill out the first draft of your tax return.