The reason is that the nWOD system just doesn't allow for it. It works off of a standardized core book for all the lines; World of Darkness Rulebook.

oWOD doesn't do this and essentially has the various game lines such as Werewolf: The Apocalypse and Vampire: The Masquerade that can be mixed, but are essentially worlds unto themselves. For example; In that world there are such things as Abominations. A Vampire Werewolf hybrid. These are Embraced Werewolves that both possess the power of Gnosis an Kindred Vitae. But even the books never state that those werewolves are the same ones in both worlds. The writer of the VTM translation guide for oWOD to nWOD even specifically makes this point:

> But in the original World of Darkness, the different game lines were ostensibly worlds unto themselves. That is, although crossover systems and products existed, it was always stated up front that the 'Lupines' of Vampire: The Masquerade weren’t necessarily the Garou of Werewolf: The Apocalypse. In Requiem, this isn’t true. The core rules of the setting are found in the World of Darkness Rulebook

I'm not saying that you can't do this for nWOD, it will just take some very creative leeway with the defined set of rules. But in the end, even White-Wolf states that all the rules in the book are nothing more than suggestions and that you should have fun with it.