I think tropical islands are so desirable in part due to a lack of coastal pollution in their ocean waters. Clear water around tropical island beaches are a major attraction for tourists and people who enjoy swimming or bathing in the Ocean. Coastal water that is near major cities are very dark and do not look as appealing or relaxing as the water that surrounds tropical islands.
I do think tropical islands are so desirable in part due a lack of coastal pollution in their ocean waters. That is one of many reasons as to why they are so comfortable and aesthetically-pleasing. The biggest reason, though, is probably just the pure beauty and nice weather all year round.
Tropical islands are also desirable because they don't have cold weather whatsoever. The tropics have two seasons: a wet season and a dry season. Some places like Hawaii rarely get hurricanes or typhoons. The ideal tropical island doesn't get hurricanes, stays between 70 and 85 degrees all year, and doesn't have coastal pollution.
People desire pure waters, when you think of tropical islands, the first thing you think of is the beautiful clean coastal waters. Some are so clear you can see right through them without any trouble. Lack of pollution is a big cause for that. As more people travel to these tropical islands though, pollution will unfortunately follow.
Yes, lack of pollution is definitely part of it. When people close their eyes and imagine paradise, many of them imagine tropical beaches with clean white sand and crystal clear or pretty blue water. Pollution would definitely put a damper on that vision. I think many people also love the sun and the general way that they feel.