Access to a large electric system provides big economies of scale not only  in terms of diversity  and low cost of power generation, but even more so for minimizing necessary reserves and for making it easier to balance variable resources like wind and solar.

Islands without access to neighboring electric systems are in every aspect on their own. They have to generate all the power they need and they must themselves keep the system reliable no matter what. Adding more renewable energy to an island system can be an opportunity, but large amounts of variable renewable energy increases the challenge of maintaining reliability at reasonable costs.