Why was Stephane Guivarc'h so integral to France's 1998 triumph...?

and, a qualified success in French club football, yet an unmitigated failure in the PL and mediocre for Rangers? The disparity in the relative strengths of Le Championnat and the PL wasn't as great as it has been for the latter half of this decade, so I'm interested to see what you all think. Was it simply physicality, homesickness or does it run deeper?