The issue with a sales tax is that it taxes on consumption instead of income. Consumption needs (as opposed to wants) stop increasing somewhere at around ~100k/yr (adjust for location). A high earner is incentivized to save most of their money and become wealthy to avoid tax.

Assuming 15% sales tax, no income tax collection.

Someone who makes 100k per year and spends nearly all pays ~15k via this sales tax or 15%.

Someone who makes 500k per year and spends 100k pays ~15k via this sales tax or 3%.

Someone who makes 10mm per year and spends 1mm pays $150k via this sales tax or 1.5%.

The lower income person pays a much higher rate, because they have to.