YEC presents a challenge because only the best tennis players participate there. As we all know Nadal is hardly ever capable of going through several quality opponents in succession outside of clay. There are no evaporated draws in YEC so he can't fluke a title, and that is because he has always sucked indoors, not because YEC is biased towards hard court players. You have a three month long clay season and barely any indoor events and you are still moaning about YEC not being played on clay, how triggered are you LOL.
The same reason why he thinks Federer would have zero clay WTF titles when he's won Roland Garros and several clay Masters.
This post (or any pointing at the same problem) deserves a special attention.
It accentuates an interesting side of Nadal as a competitor and a tennis player.
It has been discussed many times about the resolve with which Nadal overcomes challenges.
That is where the myth (yes, it is a myth) about Nadal's mental fortitude and tennis intelligence started: that because he has that inane ability to take down any competitor when the time is right (that is another can of worms) he has those in spades and some even consider him the best in those areas.
The lack of success at the WTF presents the question: is that really so, or is Nadal a conjurer of fake impressions, a mental fortress when all is in his favour and the reputation of him being intelligent in tennis terms
is a construct based on his success?
The thoughts I am entertaining in this regard are that Nadal is extremely capable of learning and forgetting patterns.
He is able to learn how to play against any competitor and, as long as it is not a long perspective, he can "groove" his strengths into the patterns he needs to execute to beat a certain style of playing/particular game. It doesn't become engrained in his understanding about how to play tennis, that is why it is not sustainable in the long run.
Now, what does that have to do with him not winning the WTF?
The thing is, WTF is a place, where not only there are very strong competitors (In itself that would not have been a problem, as Nadal has proven without a doubt that he can reach very high levels of competitiveness), but there are also very different competitors.
He needs to focus on each differently and that takes a lot of mental strength and energy to execute properly.
He also needs to maintain a high enough level of slightly or vastly different playing patterns and while this is not uncommon in tournaments where a player meets different players with different styles the difference is that at the WTF (this to lesser extent is true for the later stages of the Majors) he needs to do it consistently on a much higher level, whereas in the early stages of the Majors and in the lesser tournaments the difference in the level of the quality of the players can carry the day.