Various aspects which you can combine in various ways.
The people who live on that island wish to remain independent. They'll put up strong resistance. They'll lose, but victory will be costly for the invader, and the resources consumed would be better spent on directly confronting the enemy. Note that invading an island is always expensive. You need a fleet of amphibious vehicles. You need to plan for many more casualties than the defenders, because invaders are exposed on beaches and defenders are well dug in. You need to supply your invaders.
The people who live on the island will immediately ally themselves with the other superpower, and given such military support they could be worse than simply a distraction. So neither superpower wants to cause the island's population to fight for the other side (with support). Better they stay neutral.
Any military presence on that island would be perceived as a direct military threat by a significant third neutral power, so whichever superpower takes the island may tip the military balance against itself, by bringing the third power into the conflict on the other side, or merely by finding that the third power imposes export or financial sanctions.
There are no resources on the island worth the cost of invading it. Include under resources, location itself. In other words, the island might be an "unsinkable aircraft carrier", but its in a completely useless location from this perspective.
There is known to be an endemic parasite on the island, to which the indigenous islanders have a degree of resistance born of long suffering and evolution. It's infectious. Think, maybe, malaria, but capable of spreading via bodily contact rather than an insect vector. Anyway, a very good reason not to go there without supplies of a hard-to-manufacture, expensive, and not entirely reliable prophylactic drug.
The island is protected by alternating weather systems. The weather will turn against either invading superpower six months later, making its supply route hard and the other side's attack route easier.
(SF answer with acknowledgement to the late Iain M Banks) The island looks to be a prize worth taking. Except that it is also known from history, that the last five civilisations which attacked it did not prosper in the long term. Circumstantially, the island appears to have something of the nature of a local deity or force of nature protecting it. So, are we feeling lucky?