The Wasteland is the way it is because it is tied spiritually/magically/metaphorically to an injury suffered by the Fisher King. The Fisher King, stuck in a stasis that is prolonging his suffering without the ability to either heal or die, has brought on the Wasteland. His injury is usually connected to his virility–he might have gotten too old, or he might have been wounded in groin or leg, leaving him crippled. In any case, if he is infertile, so is his kingdom. He needs a Grail Knight to break the stasis, allowing the Fisher King to die.
The Wasteland part of Mad Max: Fury Road is pretty easy. Identifying the Fisher King(s) of the film is harder, more complex. Of course, contemporary versions of Fisher King mythology often take liberties with the story. Heck, that was true centuries ago when the mythology was getting fleshed out in the medieval romances.
Fury Road appears to have two Fisher Kings: Immortan Joe and Max.
First, the Fisher King has to be ruler or representative of the Wasteland. So, the obvious choice is Immortan Joe, and certainly he is infirm with his need for oxygen mask and protective armor. He rules the Citadel oblivious to the suffering of his people. Traditionally, the Fisher King lives in an otherworldly castle that is under a spell of stasis. The Fisher King’s nobility feast and drink while outside the people suffer. Certainly the Citadel is an otherworldly place on the landscape of Mad Max: Fury Road. It appears nearly magical when the water gets poured on the people and when we get shots of the greenhouses. Joe has chosen a handful of men to share in his wealth, and they ignore the suffering of those under them. Though he is clearly still virile, since he is the father of the unborn baby the pregnant wife carries, it almost seems as if he’s hoarded up the fertility to keep it for his tight circle, thus creating the Wasteland.
Max, on the other hand, doesn’t rule the Wasteland, but he is nearly a legend of it, at least to the Mad Max-watching audience. More aptly, he has the leg injury that makes him limp. The film repeatedly references his feet and shoes, emphasizing that part of his body (Children of Men also does this). He loses a boot, which is picked up by Nux, and which he reclaims later. His stasis is his post-traumatic stress disorder, which keeps him reliving the past events that wounded him. The flashes he gets of the little girl needing to be saved are also metaphorical. The little girl is a symbol of fertility needing to be protected and saved.
In some versions of the Fisher King story, there are two Fisher Kings–one who remains in the Grail castle, and one who appears outside of the castle and mentors the Grail Knight. I suspect what we have here is something like that, though admittedly Joe leaves the castle. But Max clearly has a direct connection to the Grail Knight and plays an active hand in killing Joe, thus breaking the stasis and returning fertility (both in the form of the wives and the seed bag).