Knowledge and ideas matter more and more. Labor in the global economy is highly mobile. It is one thing to rely on an income tax when people work in factories and taxes are deducted monthly. But the income tax was an Industrial Age solution meant for Industrial Age employers and family structure. It will be much more difficult to rely on an income tax when entrepreneurs are multiplying by the millions.
Compliance costs are already too high.
They will get worse as the society evolves. Not only will it be necessary to enlarge the Internal Revenue Service as people find more and more ways to create wealth, but the IRS will become more intrusive and its procedures more subject to abuse than at present as it struggles to trace income. But that scenario is disappearing. On the other hand, with a national sales tax there would be no need for the government to know the amount of a person's income. For purposes of taxation, it would be irrelevant.