Tech log will giver better answers.
In a v brief nutshell, all low vis procedures decision height are based on rad alt.This therefore requires a level ground surface from the threshold to quite a way out on the approach. Jersey has St Peters valley on short finals to 27, and a dirty great cliff for 09. Both ends therefore have rapidly changinging rad alt valeues just prior to touchdown. It will stay Cat I capable until HUD displays are common place and certified for everyday use.
Hope this helps.