Wikipedia has a pretty good article on it, which echoes everything I've ever been told on the subject: http://en.wikipedia.org/wiki/Radial_engine A relevant portion:
I always assumed it was just because the radial engines were air-cooled. Not a lot of fresh water in the middle of the ocean. Over land in the Pacific and in China where fresh water was available, water cooled Mustangs and P40 Tomahawks (the Flying Tigers) etc were used.
My assumption never got that far into the technicalities sorry! I've always assumed a radiator is the weak point of any engine when it comes to bullets and flak etc and springing a leak. Like those old gangster cars blowing steam when they get hit by the Bonnie & Clyde machine guns.