Then there's the consideration of keys that are naturally set for strings.

The violin's strings are G, D, A, and E, a fifth apart. Those keys, both major and minor, are easiest to play for the violinist--or perhaps a better way to state this would be, they're more available. And there are countless works set around those keys.

It works for the cello and viola, too, though there's a little difference here:

C, G, D, and A being the tunings of their strings.

I don't offer the above to suggest that other keys are avoided in any way--instead, just to point out that a great deal of the literature is built around those keys because of how the string instruments are set up.