A sales tax would not be more equitable than the federal income tax because it would hurt the poor the most.
There are basic needs that every person, rich or poor, must have. To tax heavily the basic needs and services that everyone must purchase is to place an undue burden on the poor. Certain basic things would become completely unaffordable for those who didn't have much money. The rich and poor would basically be paying the same taxes, but it would cripple the poor while not affecting the richest. This is not a humane concept.