My guess would be because it is a price that is easy to know, and one that would push shoppers away. I know that a gallon of milk is 4.99 at most grocery stores(where I live in Canada). I don't know off the top of my head the exact price for any of the other things I regularly buy. If I saw a gallon of milk for 5.99 I would assume that I was shopping for groceries at a place that has generally higher prices for all foods. I doubt this is a rational signal, but it is the way that my brain works.