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

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.

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.