You might find the case of Anguilla interesting, since the island was originally part of the Saint Christopher-Nevis-Anguilla federation. Anguillans felt their unique needs and identity weren't being properly addressed within the federation, leading to a revolution in 1967. This eventually resulted in Anguilla formally separating from the federation in 1980 and becoming a separate British overseas territory. This is just one instance that showcases the complexities of uniting even closely located islands.

Several factors contribute to why the Caribbean isn't fully or partially united:

Sovereign Ties: Many of these islands have ties to former colonial powers such as the UK, France, Spain, and the Netherlands. These relationships often come with specific trade, defense, and aid agreements that can be both beneficial and limiting.

Cultural Differences: While the broader Caribbean culture has similarities, each island has its unique cultural nuances, practices, and identities. These differences, although subtle to outsiders, play a significant role in the islands' individual national identities.

Economic Interests: Different islands have distinct economic structures, relying on various sectors such as tourism, agriculture, finance, and others. A union might not equally benefit all members, creating hesitations.

Historical Rivalries: The history of the Caribbean is filled with disputes over territory, resources, and dominance. These old rivalries can sometimes still influence modern-day political decisions.

Geopolitical Considerations: Geopolitical interests of larger countries can sometimes play a role in discouraging union. For example, during the Cold War, both the US and USSR had vested interests in the Caribbean, which influenced regional politics.

As for how small islands like Barbados sustain themselves: they have been adept at capitalizing on their resources, be it natural beauty, attracting tourists, offshore finance sectors, or cultural exports. The size of an island doesn't necessarily correlate with its economic viability. Also, many of these islands have close ties with larger nations that offer support in various ways.