Easy, actually. Unless you're chartering a yacht or similar, I can't think of other places in Thailand that have the same level of combo of luxury combined with a nice, quiet beach. And also because it combines with or can be a part of an itinerary combining places like Yao Noi or Krabi etc.
Also, and maybe this is going off topic a bit, but I found that the best food in Phuket has largely been at the top end resorts (rather than outside or streetside or wherever), whereas for Koh Samui it's the reverse.