I think you'll find several reasons why diesels haven't become so popular for sport boats, at least in the US.
Sound.
For a sport boat, that's a lot of it for many people. Diesels just don't sound as good.
Perception.
True or not, most people in the US are ignorant about diesel. Most still think of the diesels of the 70s and 80s and don't know anything about the current state of the art.
Price.
Diesels have gotten much better over the years, but they haven't exactly gotten cheaper. And of course for that big torque at low RPM, you'll need some specialized running gear. Yanmar, Merc, and Volvo all put sterndrives behind diesels, but not in performance applications.
Diesels have gotten a lot better as regards performance applications with lighter weight, smaller packages, and higher RPM, but they still don't match gas engines in small performance boats. I also think you'd find the fuel mileage wouldn't be that much higher. Better, yes, but not double as many people think. I would guess 25% improvement, but with a corresponding loss of performance.
Of course, as you move up in size the more sense diesels make. By the time you get up to something around 40', diesel is a good option even with current power choices.
I would love to see some new diesel development though. Take the BMW 3.0 diesel as used in a cars like the 335d and 535d. That could be an interesting engine if marinized, and I'm sure there are other performance diesels on the roads in Europe that we don't get here that might make good platforms for smaller sport boats. It would certainly be interesting to try.
Don
'01 22 Classic, 502/B1
And a bunch of other stuff
"If everything seems under control, you're just not going fast enough" - Mario Andretti