It is a lot more than just the one shot advantage.
The leaf spring trigger has a very different feel from that of the coil spring trigger.
Leverage for manipulating the trigger is better on the K frame.
Recoil is softer on the K frame.
Those are not minor advantages. The only real advantages of the J frame are concealability and weight.
The J frames aren't bad shooters, but the K frames are more user friendly.