It's a monopoly.....sort of.
Merc had the resources and money to develop, test, and market the drive system. Even though Volvo came out with it first. That's why only Merc and Volvo are in competition.
Since Merc has the major share of the boating industry, and Volvo the rest, others may not want to spend the money required to try and beat Merc at their own game for small piece of the pie.
Polaris stopped production of all watercraft for similar reasons. They were always number four in the market and no matter how had they tried the couldn't beat SeaDoo, Yamaha, and Kawasaki. Even though Polaris won watercraft of the year with their new model and they were just about to introduce a line of boats like SeaDoo and Yamaha, production immediately stopped.