Originally Posted by Thracozaag
I think with tennis (and to an extent golf as well), the modern equipment is certainly a factor.
I think this is only true to a limited extent, but it's becoming more of a factor--many of the younger players even now honed their strokes on older, more flexible frames and still tend to prefer them for their consistency, complete with natural gut (which hasn't changed so very much in the last decade), often strung at tensions that exceed the manufacturer's specs and further dilute what little inherent power the frame might have had. This also tells you how hard the current players hit to generate the power that they do.
Some (particularly women) have switched to widebodies and this trend will likely continue, but I think the bigger factors are the more serious (and technologically advanced) conditioning regimes, better footwear technology, and faster courts.