We have to ask composers. You don't have to be able to play a certain instrument and write freakishly difficult music for it. For example this russian Composer didn't know how to play the Cello and still wrote some of the most demanding pieces/concerto for it: https://www.youtube.com/watch?v=OFYz6Bn-3T0 . It is not a technical problem, you don't have to be a virtuoso, you need other qualities. Imagination, simply a feeling of what people will enjoy, good taste and endurance. A brilliant composer who is able to write really good music, be it easy to play or hard also has to be a genius.