Since the time of Beethoven, writing a Ninth Symphony has proven a daunting prospect 

for composers.  Not only is Beethoven’s Ninth an unmatchable work, but there seems to be 

something fatal about Ninth Symphonies: Beethoven, Schubert, Dvorak, Vaughan Williams, and Sessions wrote only nine, and Bruckner died while writing his Ninth.  Mahler in particular was superstitious about writing a Ninth Symphony and regarded it as a potentially deadly act.  After completing his Eighth, Mahler composed Das Lied von der Erde, and once that was done and he had begun his Ninth, he claimed that Das Lied was a vocal symphony and so he had beaten the curse and was writing what would actually be his Tenth.  Mahler had reason to be concerned: his Ninth turned out to be his final completed work, and he died before hearing a note of it.  Given this heritage, it is not surprising that composers have been wary of writing a Ninth Symphony.