A different option would be one ruleset (to bind them all to....oh wrong system but that just had to be

)
It's not really a question of rulesets -- though that matters some, also, with nWoD 2e rules being
a lot more modern and supportive of narrative themes.
It's a question of totally different
games. nWoD Mage is absolutely, totally different from cWoD Mage, as are Demon and Changeling (at the least). There simply isn't any way to "bring them together" without totally rewriting the core themes and concepts in those games (i.e. without changing them into
totally different games). Guess how much fans would like that? Yeah. To add to the mix, there are lots of people who like, say,
both variants (Mage, for example), for very different reasons. Because, again, they are completely different games, on fundamental levels.
Mage: the Ascension has moldable reality, based on consensus, with the Technocracy (another group of Mages) as default antagonists. Mage: the Awakening has static reality, with it being a Gnostic prison, ruled over by the Exarchs, godlike jailers, and their minions. Ascension has sphere-based, semi-freeform magic. Awakening has (mostly) Hermetic magic. Other than the word "Mage", there is precious little common ground.
Demon: the Fallen has Judeo-Christian fallen angels. Demon: the Descent has weird, techno-magic constructs of a vast, uncaring God-Machine, with reality as a form of Matrix. Again, not much common ground.
Changeling: the Dreaming has its root in fairy tales and stories, and in keeping (mostly bright and happy) fantasy alive. Changeling: the Lost is a grim tale about survivors of abuse at the hands of supernatural beings, with lots of heavy real-world "abuse survivor" themes. Again, no common ground.
Vampire and Werewolf are most similar. There, some sort of merge is at least vaguely possible. The others? Forget about it.