The improved speed, is definitely a factor. But, I think reliability also played into it. I have worked on a lot of old front action locks, with major internal problems, but very few back action locks suffering such maladies. The wrist breakage is very common in guns with bask action locks. A lot of this is due to the lock not being properly fit to the snail, or drum. Any movement of the lock when the hammer falls, puts a lot of strain on the already weakened wrist.
Hungry Horse