Several reasons imo.
- Nadals biggest strengths are the ones which decline the most with age. He has changed his style which has been succesful vs the field, but not Djoker/Fed. I dont understand those saying Nadal is prime. There is no way prime Nadal would go 0-12 vs Djoko/Fed off-clay (sinse 2015)
- The obvious one, clay. Their H2H was very clay skewed early days, and Nadal usually didnt go deep enough off-clay to face Rog.
- Roger was a bit unlucky with their meatings form-wise pre2015 (like their only Cincy-match 2013, Rogers worst year ever). From 2015- its been the opposite.
- Their H2H is weird. Skewed towards clay, skewed towards early season. No USO in 15 years, etc...