Jon really blew his voice out on the Slippery When Wet tour prior to that (check out Landover 1987 on my channel - it's a clean Soundboard capture that gives you an idea of just how far his voice had already gone to hell). He always went to the absolute limits of his then-current vocal capabilities in the studio and I think these two factors combined became the reason why they tuned the songs down from 1988 onwards. Basically to help him through long tours and preserver the vocal chords better. I know the Scorpions started doing similar things at some point, but then again it was around the time their singer was in mid-60s and being able to pull off a power ballad like Still Loving You in any way at that age is a tough thing to do :)