Why not just charter multiple yachts in Greece? You can, but it changes the trip and the math. Greek island groups, the Cyclades, the Ionian, the Saronic, spread out with longer crossings between stops, often two to three hours or more compared to Croatia's tighter chain of islands. Splitting a group of twenty across two or three yachts means coordinating separate captains, separate schedules, and separate itineraries that rarely land in the same port at the same time.
There is also a cost gap working against Greece. At equivalent yacht size and tier, Croatia runs roughly 20 to 30% lower than Greek waters in base charter fees, and Croatia's islands sit close enough together that a single vessel can comfortably cover a week's itinerary without long open-water passages. Groups get the same ultra-luxury experience at a meaningfully lower cost, on one boat, with one consistent crew for the whole trip.
For a group booking specifically, that difference is not a minor detail. It is the difference between one seamless product and three coordinated ones.