Some 20 years or so ago, I spent two days with the FAA (on loan) in the Hatfield BAe146 simulator flying lots and lots of approaches into the likes of Aspen, Colorado and Casper, Wyoming. The object of the exercise was to establish the steepest safe approach possible.
If we got everything exactly right at the beginning of the approach, a 7 degree glideslope with everything hanging out at glideslope capture was possible but if you got high or fast then there was absolutely nothing you could do to correct.
It was decided that using a 7 degree slope on an everyday basis could be fraught with disaster.
I believe that was one of the reasons that a slope of 5.5 degrees was decided upon at LCY.