Diet Pepsi seems more common in Negril than Diet Coke or Coke 'Light".
On an island with a huge crop of cane sugar, drinks tend to contain lots of that; even orange juice is often sweetened with sugar.
Maybe the abundance of that ingredient makes soft drinks cheaper or perhaps the median income of the population dictates a lower price.
Cell Phones are ridiculously cheap by North American standards. A smaller "grid" may be part of the reason for that but everyone and their brother seems to have a cellie.