Bolt and Phelps lack impact at a cultural level, agreed. Running and Swimming are the most primal/basic sports and still these guys cannot become that epic. In Bolt's defense I would say he is Jamaican, he isn't an American, had he been an American then he would have got more opportunity to grow, his english would be better, he is already the fastest man alive - we would have seen him in hollywood movies. His legend would have grown.
Arnold Schwarzenegger who is an austrian and speaks english in a funny accent is rated bodybuilding GOAT and action GOAT (along with Stallone) because of Hollywood, now imagine if Arnold never ventured into Hollywood? He would be lesser known than Ronnie Coleman and Jay Cutler, both of those guys are legends and no less than Arnie in bodybuilding but Hollywood makes a difference. Even Bruce Lee is hyped because of Bollywood..... Bolt could have been big if he was an American and probably a bit better looking too. He looks ugly, and ugly means ugly, it is not about skin color, it is about looks, we have action stars like Michael Jay White or Idris Alba or Will Smith making it big but Usain cannot because he is not good looking too. So these things matter. Phelps I think has even lesser excuses than Bolt..... Phelps is american and yet he is not that iconic....Phelps is worse than Usain.
In Djokovic's defense, I would say that his biggest DRAWBACK is his loser status in USA, that means his loser performance in US Open finals..... I donno how the mindset of an American works (you would know better) but I can imagine that the crowd has only seen this fellow lose finals to Federer, then to Nadal and latest even to Medvedev, WTF ... I am not even counting Murry/Stan, but look at the picture, he is losing so much and we live in a world where the most hyped athletes are in USA so in a land where the spotlight is on you, you are a flop... that holds against you.... Novak has somehow won 4 US Opens, good for him but audience has seen him as a loser in USA for a long time .... Good for him that he has 7 wimbledons, can you imagine his stature if he had lets say 3 wimbledons, 4 us opens, 3 french opens and 14 aus opens ??? People would not take him as seriously as he is now because then he would be average in USA and Eng and these are important Slams.....
So thats that....