Cold War politics intersecting with domestic politics. The CIA got into bed with the JLP and PNP to keep them away from communism. And the two parties had gangsters on their pay rolls like George Feathermop Spence, and Claudius Massop (who was the original leader of the Shower Posse). These guys prior to Cold War tensions were more small time enforcers for their political overlords, giving votes in exchange for government contracts and cash. But after the CIA started getting involved in Jamaican politics they started to get guns too. Really good guns at that. Pretty soon they became so powerful they got out of the politicians pockets.
At the same time the CIA was also getting involved with drug lords in South America. Due to its favorable location between South America, North America and Europe, Jamaica became an ideal distribution centre where shipments from South America could be sorted out and forwarded to wholesalers in North America and in Europe.
So you had both a stash of high end weapons and a steady flow of big drug money coming into the country at the same time, as a result of US foreign policy colliding with the already corrupt nature of Jamaican politics. That is a recipe for a perfect storm. And that's how Jamaica became so dangerous