It's that it's easier to show a price of $0.99 and have the consumer pay $1.08 (for example) than either show a price of $1.08 and have the consumer pay it, or show a price of $0.99 and have the consumer pay $0.99 and "lose" 7 cents (because your price was $0.92 before taxes).
Pre-tax price is lower and sells better than post-tax price.