Spring steel, as it's name implies, bends.
This is an awesome attribute to have if you wanted to build a skyscraper that won't get blown over by a windstorm, or any other application where you need your metal to bend.
With picks however, that bending actually
dampens the vibrations ever so slightly, and an experienced lock picker will notice. An ultra rigid metal like titanium would be great for pick material - except it's very likely to break.
I guess it's a trade off. Dirty, dirty n00bz0rz will want spring steel so they don't break their picks too often. The seasoned ones will want to use a more ridgid metal for better feedback.