To streamline the leaderboard, the "difficulty" option has been limited to All Missions. The rationale is as follows:
(1) By definition, an Any% speedrun does not require a particular difficulty (see http://www.speedrunslive.com/faq/glossary/ ). Since the discovery of Crouch-Clipping, there have been no Any% runs (featuring Crouch-Clipping or Crouch-Clipping with Resolution Toggling) on a difficulty other than Novice.
(2) By definition, a Low% speedrun with Crouch-Clipping would be identical to the current Any% route, which collects either minimal (Crouch-Clipping Only) or no (Crouch-Clipping with Resolution Toggling) items. To avoid having the Low% category subsumed by the Any% category, it has been retained as an "All Missions" type speedrun that is played on the "Nightmare" difficulty.
(3) There have never been any Low% or 100% runs (with or without any form of Crouch-Clipping) on a difficulty other than Nightmare. Of course, this does not preclude such runs in either category, but absent at least expressed interest (if not actual runs), there is little justification for retaining difficulty (and therefore empty categories).