The in-game reason appears to be “It just doesn't work.”

Vampire: the Requiem, page 168

Kindred Vitae, while potent, does have its limits. When the other supernatural creatures with whom the Damned occasionally cross paths come into play, certain realities of the World of Darkness supersede the mystic strength of the Blood.

Werewolves and mages are immune to the Embrace. If a vampire is somehow able to extract all the blood from a werewolf or mage and attempt the Embrace, that Embrace fails automatically.

 

Mage: the Awakening, page 65

Note that a character cannot possess multiple supernatural templates... A vampire or werewolf cannot Awaken...

I suspect it doesn't work by design because they wanted to avoid the mixing madness of classic World of Darkness.

The in-game reason appears to be “It just doesn't work.”

Vampire: the Requiem, page 168

Kindred Vitae, while potent, does have its limits. When the other supernatural creatures with whom the Damned occasionally cross paths come into play, certain realities of the World of Darkness supersede the mystic strength of the Blood.

Werewolves and mages are immune to the Embrace. If a vampire is somehow able to extract all the blood from a werewolf or mage and attempt the Embrace, that Embrace fails automatically.

 

---

_Mage: the Awakening, page 65_

Note that a character cannot possess multiple supernatural templates... A vampire or werewolf cannot Awaken...

I suspect it doesn't work by design because they wanted to avoid the mixing madness of classic World of Darkness.