One thing that is interesting to me is that in all of the players listed who didn't complete their career slam, it is one of the natural surfaces that has been their doom.
Also, looking at the players, it seems there is mostly one style category of player that hasn't won Roland Garros, and one that hasn't won Wimbledon.
The serve and volley players like Sampras, Edberg, Becker, Newcombe never won the French Open.
The baseliners like Wilander, Lendl, Vilas, never won Wimbledon.
Looking at Novak's game and style in isolation, I might have predicted that he would have the most difficulty winning Wimbledon, though it's true that recently baseliners have had more success there due to the generally higher bouncing and consistent grass court.
But, he did win there, and it has been Roland Garros where he has failed thus far.
To me, he's most similar to Jimmy Connors in this respect. Both aggressive baseliners who were/are excellent on clay.
Of course, Connors was prevented from playing at Roland Garros in 5 of his best years due to his contract with WTT and thereafter, Borg, Lendl, and other clay adepts got in his way, whereas Djokovic has had Nadal and a few others blocking him thus far.
Of course, a little luck never hurts when there aren't too many standing in your way. 
Respectfully,
masterclass
great post and great observations.
the game today is different. all warfare on the battlefield has to be waged from the backcourt.
so it will be the baseliners winning everything. serve n volley players would not be able to put food on the table in modern tennis unless you were bryan brothers and that is doubles.
stan hurt roger from the backcourt yesterday. he pressed and attacked roger's weaker wing from the backcourt. roger would go on to make as many as 18 unforced errors off that wing. the war has to be and must be waged from the backcourt. going forward entails a lot of risk today so you can only go in on your own terms.
but that being said, all your observations are correct. natural surfaces have been a bit of a problem for some players. as good as they were on quicker grass, they found themselves a bit limited on the red clay.
newcombe, Sampras, and johnny mac were never able to win at RG.