I think what it boils down to is that the Volvo Penta and The GM Vortec platforms along with the Mercruiser stuff are proven marine setups that are reasonably reliable and, most importantly, are easy to get marine parts for. The consumer market is driven by the industry and where the industry wants to go.
I remember when racing jet skiis that stand up was the premier event in the IJSBA. Over the course of a couple of years, the industry pushed it to the runabouts. Why? Beacause they can sell more runabouts. Stand ups almost have gone the way of the Do-Do.
I believe a nice Ford Racing 514 Stroker would make a hell of a marine engine. Unfortunately, by the time you got one turning a prop, you probably could have bought 2 big block chevys