Culture (inertia) is hard to change ... though it seems to me that all SAR ops are moving towards twins, particularly if Category-A design may be a consideration (JAR-OPS requirement?).
Operating costs of twins would certainly be an issue...
Even with today's engines, a engine failure during OGE/hoisting ops at a high DA could be just as ugly as having only one engine. But if the engine failure occurred during any other phase of flight, then a twin is the way to go... I'll take the twin!
[/random thoughts]