Supernatural power comes from making mutually exclusive changes to the human soul.
My view isn't explicitly stated anywhere, but I consider it supported by statements across the nWoD books. There are a few representative quotes from the core books I own at the bottom.
To do anything majorly supernatural, you need to be more than human. A vampire's soul is deadened and blended with the Beast from his sire's vitae. Werewolves are born with a soul that is supernaturally potent but sleeping for years. Mages awaken and gain sympathy between their souls and the Realms Supernal. Changelings are torn, mangled and blended with Fae stuff.
None of these changes can coexist in one person. Taking a Mage for an example. If you aren't a normal mortal, then you cannot be in close enough sympathy with the supernal realms to perform magic.
From Vampire The Requiem 181
The change from life to undeath affects more than a person's body. It changes the soul.
From Werewolf the Forsaken Page 75
A werewolf is born supernatural, with the soul of an ancient predator.
From Mage the Awakening Page 32
A mage who loses his soul immediately enters Sleep until it is restored. His ability to perform magic is gone. If the mage somehow gets his soul back, he needs a full night's sleep before he can use magic again.
From Changeling the Lost Page 24
When humans enter the Hedge, the Thorns do more than tear at their flesh — the Thorns rip away at their souls as well.
While the human soul is not quantifiable, there is a certain something which makes humanity humanity, and it is this same unique characteristic that begins to unravel like a mis-kint sweater the deeper one travels into Fae lands.