Gustav Styger ask:

>Does anyone know the reason for this, or might there be a lost masterpiece
somewhere?

Mozart always wrote pieces for good soloists.

It was for piano (himself and others), clarinet, bassoon etc.  He never
knew a good cellist.  Rarely the instrument was used in soloist passages:
it doubled the contrabass in the orchestra, and it played only the
accompaniment in the string quartet.  Moreover the utilization of the
spike - to stay the instrument in the ground to free the resonance and
to enrich the timbre - was not usual in that times in all places of the
Europe.  Only Haydn and Boccherini wrote very good for cello in that
age.

Edson Tadeu Ortolan
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http://sites.uol.com.br/eortolan