Where do knife steels come from?

[me2]

Spring steel is a vague term, perhaps even more so than surgical stainless as Mark mentioned.   There are a staggering variety of springs and materials used to make them.  The main requirement is sufficient strength to prevent bending in the chosen application.  A secondary consideration is the fatigue life of the material.  I suspect this is the reason for choosing medium carbon steels for many applications, as the carbides found in higher carbon steels might act as initiation points for fatigue cracks.  Or it could just be these are often more expensive.  

5160 is often used in automotive support applications.  1095 is used in tape measures.  301 stainless is used in some applications, and makes a fine spring once cold rolled or drawn into wire.  

Higher hardness makes a "better" spring, but there is a point off diminishing returns.  A spring with too high a hardness will break if pushed beyond its limit, where a proper spring will bend.  Try to roll a high hardness edge and it will deflect further and still return to straight.  However, push it to hard and it will break/chip.  A softer edge will just roll over and can possibly be straightened.

Edges behave oddly to most of us because they are one of the few things we deal with on such a small scale, as in the edge thickness of a sharp edge is less than 1 micron.