Why is Pibb Zero so hard to find?

This might be a dumb question, but I’ve been trying to find Pibb Zero lately here in North Texas. It actually seems to exist here, because I found my local Kroger was fully stocked and I bought like 4 cases, but it isn’t ever available in Mississippi where I go to college.

Do y’all know why it might be difficult to find in different states? Or why it’s difficult to find in general? Is it due to production issues, people don’t drink it, etc.?