Probably because it's not true. Of course, it all depends also on definition of "tax rate" - are you talking about federal income tax? All taxes combined? Marginal tax rate? Effective tax rate? Single-year or averaged over a period of time? Do you include corporate taxes (which are paid by corporation, but effectively reduce the wealth of corporate owner or shareholder)? Until you define all those, this slogan is meaningless.
> I'm not going to discuss about finances with Warren Buffet.
He wasn't discussing finance, he was propagandizing. These are completely different games with completely different rules. In finance, numbers and facts matter - if you lie in a financial report, you risk heavy fines or jail, and probably will lose confidence of your business partners. If you lie in propaganda, as long as the lie is believable, well packaged and matches the prejudices of your target audience, you win.
He also said something in the lines that "he was paying a lower tax rate than his secretary". That sounds crazy to me, but I'm not going to discuss about finances with Warren Buffet.