Good question Guptar, from my experience in Beavers and other radial engined aircraft, I suggest it may have a bit to do with the Pilot.
Radials require very careful engine management right from the pre flight (Oil in the lower cylinders) through to the shut down. I do not believe that current training methods teach the correct disciplines that we were taught when I first flew in the 60's.
I have always felt that a radial is a very robust power plant in the right hands or they would not have built as many as they did?