The issue is skillset & the perceived gains PED's give athletes in their respective disciplines. 
I don't mean to generalize here (but I will, obviously), i.e. 'people' have an overly simplistic view whereby they equate tennis, football & other sports (mostly without drug scandals because the testing is non-existent or a joke) as skill based whilst cycling as an endurance sport is seen as fertile ground for cheats who win only because they dope & without drugs would be worthless. 
Nadal is viewed as a genius who even when faced with accusations of doping, he's seen as skilled to the extent the dope doesn't matter (or wouldn't make a difference). That's the basic context. 
Of course try telling people drugs absolutely make a difference in a 4 hour tennis match with endless rallies, or when football teams run, run & run forever without tiring from one match to another. 
We're in an era of Cyborgs in all sports & only one (cycling) was seriously targeted to the extent its biggest champions are dope memes every summer. This is obviously not fair at all but public opinion is a difficult beast to figure out or push in the correct direction (which in our case would be simply asking for some realism with regards to the billionaire sports of football + tennis & a little less hypocrisy towards cycling).
For example, apparently Lance Armstrong ran the "most sophisticated doping scheme in sport". Trololol etc. I mean during his era EPO could be purchased over the counter in a pharmacy ffs.