Stores have items on sale to get you into the store. You buy the sale items, but also pick up the other stuff you need. In the end, they sell more overall “stuff.”
The items that alternate (soda, cigarettes, beer) tend to be products that have very high brand-loyalty. Coke drinkers almost exclusively drink Coke. Same for Pepsi drinkers, Bud drinkers, Marlboro smokers, etc. So one week the Coke drinkers come to save on soda, the next week, the Pepsi drinkers come in. The reason they alternate, I suspect, has more to do with the suppliers than the stores. Coke wants to win over Pepsi drinkers, Pepsi wants to win over Coke drinkers (side note - ever notice how Pepsi One ads compare it to the taste of Coke and the low calories of Diet Coke?). If they’re both on sale at the same time, everyone just goes with their favorite all the time. Price wars ensue, and neither wins.
But, no one really pays that much attention to what’s on sale when. They just keep going to the same store as long as some of the stuff they buy is on sale some of the time. The there’s people like me (and I assume by your OP, you) who don’t care whether they drink Pepsi or Coke. We just buy whichever is on sale, but we also buy our other “stuff.”