One major reason I've seen is that soft actuators are complicated to use. You can't move things to a predictable location because they'll bounce, stretch as they age, and any weight changes (which applies all the time to all double+-jointed things, as e.g. straightening your elbow increases the force needed at your shoulder) change all of the above. The continuous feedback needed to counteract this is expensive and imperfect (as are we).

Existing servos / stepper motors / etc obviously do this too, but usually to a much smaller and more predictable degree.

Obviously it still has uses, e.g. as a grasper at the end of a normal robot for a softer grip. But making a whole robot out of it is still largely insane, unless you have esoteric needs. Hence ongoing research, but not application :)