Why was Richard Strauss’s Horn Concerto No. 1 especially difficult for soloists?

Richard Strauss’s Horn Concerto No. 1 in E-flat Major is considered one of the most demanding solo works for the French horn primarily due to its technical range. The composition requires the soloist to navigate the instrument’s highest and lowest registers, often moving between them in quick succession.1

The work was originally intended for the composer's father, Franz Strauss, who was a principal hornist. However, even he found the concerto extraordinarily difficult; he deemed the frequent, high-register passages too risky to perform in public and ultimately declined to premiere the piece.1