I've been thinking about this quite heavily, but if we speculate that man was created by Lorkhan, this explains a lot of things. If this is true, then men are the only ones capable of becoming Shezarrines and it may be possible to create artificial Shezarrines, as each man would have a miniscule part of Lorkhan's soul. If one were to gather enough of the souls and combine it with their own, they may become Shezarrine. Perhaps this is why black soul gems can only contain the souls of men and mer, as both of these have some divine aspect to their soul.
they have just as much divine essence as mudcrabs or slaughterfish. they revere sithis, yes, but they were not directly created by him, they were created by the hist, and the origin of the hist is unclear
Mudcrabs, and the rest of the animals, are the Earth Bones that lost so much of their divinity that they lost their intelligence. This is the basis for white souls, mostly. This also explains why Falmer have white souls instead of black souls as well, as they have fallen far from their divinity.
Man and Mer are descended from Ehlnofey. The Ehlnofey were either (A) et'ada that gave themselves to Nirn, lost their divinity, but not enough to lose themselves or (B) from one of the 12 worlds of creation or (C) from another Dream (could be B and C).
Hist are similar to Ehlnofey except they were never et'ada. They are from another Dream and ride Memory from Dream to Dream in order to continue their existence. Argonians are extentions of the Hist. Their not created by the Hist. They're more like limbs of the Hist. They're more a part of the trees than they are from them. This had always been theory but dialog from ESO has confirmed it.
I'd say this theory holds some weight with one minor problem. Any mortal can be a Shezarrine. In KotN you mantle Pelinal, a Shezarrine, and therefore become Shezarrine. There's also potential for the Last Dragonborn to be Shezarine as well. At the very least, KotN proves that any race can be Shezarrine, not just Man.