This makes total evolutionary sense: Traditionally-built turtles (thank you, Ladies' Detective Agency) don't need to invest resources in speed, soft-shell turtles do. Soft-shell turtles also can't withdraw their limbs and head into the shell.
I caught a soft shell turtle once while fishing in the Iowa River. It was purely by accident, he stepped on the hook, so I caught him through the foot. He was not a happy camper. (understandably) Fortunately I was able to just clip off the barb, and remove the hook without injuring him further. But I have to say, when I set him down, I was rather surprised at how quickly he ran off. I don't think I could have caught him again if I had wanted to. But it was really neat to get to see him up close like that.
Soft-shell turtles also can't withdraw their limbs and head into the shell.