Or because people can't get past the first 5 minutes.
The Busoni has always had a reputation for excessive difficulty, but that seems overrated to me. Perhaps it is more a matter of endurance, certainly there isn't anything in particular which puts a Brahms Bb, Rachmaninov 3, or Bartok 2 to shame.
I have loved this music since a lad of 15, and IMO no one but John Ogdon has ever really done this music justice- and I have 8 recordings including the live Mewton-Wood.
Busoni said he could account for EVERY note of that concerto, and indeed there is not a wasted moment, it is as tightly constructed as anything by Mahler, Strauss or Elgar. The problem -if it be so- is that Busoni's themes are not, by themselves, very brief. Busoni needs time (and the opening salvo is as short as it could be) to develop, but we don't see that as a problem with Mahler or Elgar, do we?
Well maybe we do, and perhaps Busoni is setting himself up for comparison with that taught conversation taking place in Beethoven's Bb concerto.