“A regressive tax is a tax imposed in such a manner that the tax rate decreases as the amount subject to taxation increases”
I find it sometimes helpful to read past the first sentence of an article. How about the second paragraph:
A value-added tax or other sales tax on food and other essentials such as clothing, transport, and residential rents can be regressive. Since the income elasticity of demand of food is usually less than 1 (see Engel’s law), it tends to take up a higher percentage of the budget of a person or family with a lower income.
Elasticity of demand is key to understanding why sales tax on many common items is regressive. The option not to purchase something is not always a valid one.