Matchstick,
I feel the answer may be , its a pretty Bullet proof engine, been around for many years, so has much history of service/ failures history of such things all help to make it the engine of choice for the R22. like its big (6Cyl) brother engine from the same maker and design is used for the R44.
Also both engines are already cleared to fly by the relevant World authorities , plus spares are available all over the "Civilised" World.
To develop a new and untested engine in today's market would cost an awful lot of Shekels..!