Why is C Sharp Minor rarely used by anything else other than the Piano?

Question

Apparently C Sharp Minor is unheard of outside of the piano. Only two hugely successful symphonies have been made in this key. I was spurred on by Beethoven’s astonishing Moonlight Sonata and was surprised to see how left out this key is.

Even Johannes Brahms still felt the need to rewrite his C-sharp minor piano quartet in C minor

Wiki

That’s saying something, the King even altering a masterpiece.

I have never understood why some keys are more challenging for professionals. I did once read that sharps are better than flats for amateurs but these professional orchestras are not amateurs.

I’m doubt this is the case, however. What could be the reason that this key is not suited for anything other than the piano?

Answer 1

C-sharp minor is not a particularly rare key.

If we take the Wikipedia page Compositions by Key, count the compositions linked and turn this into a table we get the following:
(Note: this data is probably wildly inaccurate. If you can provide a link to better statistical data, please do so in a comment.)

sharps/ key     #       key     #       key     #       key     #
flats
0       C       201                     Am      91  
1       F       127     G       119     Dm      130     Em      83
2       Bb      146     D       196     Gm      94      Bm      54
3       Eb      166     A       111     Cm      117     F#m     24
4       Ab      49      E       55      Fm      58      C#m     37
5       Db      27      B       17      Bbm     24      G#m     7
6       Gb      9       F#      20      Ebm     18      D#m     3
7                       C#      4       Abm     5

The number of works in each key drops sharply as the number of sharps or flats increases above three. If one were to take only 18th century works this trend would be even more extreme. C-sharp minor is actually slightly better represented than one would expect, possibly because it was a popular key for piano works. But there is nothing special about C-sharp minor that would make it harder for other instruments.

Another interesting observation is that in major key compositions are roughly equally divided between sharps and flats. In minor keys composers strongly prefer flat keys.

Answer 2

A quick check found Beethoven's String Quartet, (op. 131) as well as a Sinfonia by Joseph Martin Kraus (VB 140), both without piano.

Sharps are less problematic for amateur string players than flats and four accidentals in a key signature are far from uncommon, so these are unlikely reasons for the key you observe as underrepresented.