What's the point of buying a 911 over a boxster or cayman? Why would anyone spend more money on a car that's slower and has the engine in the wrong place?
Cayman gt4 and Boxster Spyder are rather focused cars. Perfectly ddable, sure, but a bit intense for a lot of tastes. GT4's manual only too, and I imagine so is the Spyder, which is going to put many buyers off.
Aside from that, there's the whole 911 heritage, Cayman and Boxster drivers couldn't afford a proper Porsche thing. Also, I'm fairly sure the GT4 was limited production and now sold out.
All that said, I know damn well which I'd rather have.