From the FAA AIM (section 2-1-2(b) Visual Glideslope Indicators):
PAPI angles are based on obstruction clearance +/- 10 degrees from the runway centerline, up to 4 miles from the threshold.
The ILS provides for obstruction clearance in a 3-6 degree angle on a 3 degree glide path.
Therefore, if there are obstructions in the PAPI clearance area (e.g. a tower off to the side of the rwy) that don’t exist in the narrower ILS clearance area, the PAPI will have to be a steeper angle.