If it's a certain formerly-cocaine-infused soft drink, the simple answer is that the vendor continually tests how much the market is willing to pay, and found that it will pay more.

I tried repeatedly to quit that particular product. I've tried grocery store generics. I bought a home soda maker and tried various craft soda syrups online. Nothing tastes the same, or even "close enough" for my palette.

I have managed to drink less of the stuff. I only buy the mini cans now (unless I'm treating myself to a bottle at a convenience store), and try not to have one at all on some days. But the sad fact is, as much as competition is supposed to solve this problem, there is really no alternative for people that are tuned into the taste of that particular brand, so they continue to pay out the ass for it when the vendor jacks up the price year after year.