For one thing, these Greek islands are a little piece of paradise only a few hours from Athens.
The 18 major islands ensure quite a bit of variety and that's without counting the several lesser Cyclades with their pristine beaches and laid back lifestyle.
Most island groups have a HUB island or administrative center.
Some like the Cyclades have several; Syros the capitol, with its beautiful buildings and sandy beaches, Mykonos, third most popular island in the Med and gate way to ancient Delos, Tinos the religious center of the Orthodox faith, Naxos, in its day, seat of a duchy, most fertile and largest. Green Paros rich in wine and marble.
Milos with its beautiful beaches, geologic formations and obsidian deposits exported since pre-historic times.
Incidentally, The Venus de Milo, discovered on Milos was made of marble from Paros (Parian). Beautiful Ios, where youth is wasted on the young and the young get wasted.
And last and certainly not least, romantic Santorini with its hot black sand beaches, unparalleled caldera views and prehistoric ruins reminiscent of Pompeii. For these reasons and many more the Cyclades island group is the most popular among tourists both Greek and foreign.