Islands have a lot of coastline per square kilometre of land, so they are going to have a bigger density of attractive locations to build hotels.
Typically the coastal part of Mediterranean islands was often the least desirable land, until modern tourism was invented. If you drive around Majorca you will see signs commemorating the fact that the road you are on was built by forced labour by prisoners of Franco during the civil war, and most of these are in the south-east, where all the best coves are — but until 1940 there was nothing going on here, which is why there was no decent road until then.
There is an apocryphal story of a farmer on Ibiza who had 5 sons, one of whom was not as bright as the others. When the old man died and the sons divvied up the land, the less-bright one got fobbed off with the bit by the cliffs, which was not good for grazing sheep as they tended to fall off. Then the rich visitors arrived and wanted plots of land overlooking the bay, and he was minted, while the others are still herding sheep.
Also, islands are mostly by definition physically separated to some extent from the main economy of a country. People won't set up major industries there, simply because of shipping considerations. So that means that the island is less spoiled visually (and hence more desirable as a location for tourism), but also the hotels and airports don't have to compete with high-paying unionised manufacturing jobs, so it becomes cheaper to hire people to work there, which makes your holiday cheaper.