Probably has something to do with the fact that desalinization plants are ridiculously expensive when you live on the same land mass as the factories that are building all the parts for them, and the price rises dramatically when it all has to be loaded up onto container ships, transported to you, and then built by a foreign company, and most of those island nations are incredibly broke.
The problem with desalination is that it's expensive and you can't dump the salt anywhere. It creates a brine that kills fish. That would hurt the environment and the tourism industry.
Not only that, they get a ton of rain every year. They're tropical islands. There's enough water, they just need to collect it. Desalination is a technical solution when they really just need boring infrastructure.
The problem with GDP per capita is that it conveniently overlooks the possibility that a lot of that GDP is artificially inflated thanks to money that never actually moves around and stimulates the economy, because that country is a tax haven for foreigners and shell corporations, which is pretty common in the Caribbean.
Head to Nassau and walk around sometime, let me know if you still think the Bahamas is in good financial shape.
But I agree, rainwater collection is what they’ve been doing for hundreds of years, they just need to find ways to do it more efficiently.
I'm the article they point to a lot of factors as to why this is now becoming an issue: increase urbanization, increased industries, increased tourism.
They do say collecting rainwater is probably part of the solution, but if the other things keep on growing that won't always be the case.