prime focus dish/offset dish

Hello all. I've been a member of this forum for a couple of years now, still learning new things about satellite technology. apologies if this is a very basic question: why are prime focus dish antennas preferred for C-band reception? whereas offset dish antennas for ku-band reception ? are...

what is the comparable efficiency of prime focus vs offset for fringe reception of 7w,26 east in South France? And for 28East,weakest channels? Would it providegreater stability against weather /seasonal variation? And,why are there so few offset dishes in France/ spain for example?