First, let me say that I like Sardathrion’s answer and keeping it a mystery that just works is an excellent way to go. But if you want an explanation:
They won't fight or destroy it because there are powerful entities that find the bar useful.
Vampire Princes often establish places of Elysium where outright violence is forbidden. These are stereotypically bars, nightclubs, or salons (in the fading meaning of a gathering place focused on intellectual discussion rather than a beauty parlor). With two vampires working there, a Prince might readily be persuaded to establish the place as Elysium and may even spend time there himself.
Tradition Mages may well want to study other supernatural creatures, and so find such a gathering place useful. (As a side note, this would be even stronger if you borrow the backstory from Mage: The Awakening instead of using the oWOD Mage: The Ascension. In The Awakening, studying mysteries like other supernaturals is a big part of a Mage’s life. I could easily see the Mysterium devoting vast resources to creating a few such places throughout the world so long as using focused mage sight is not considered breaking the peace.) If the general idea of Tradition Mages enjoying it is not enough for you, give your Death Changeling some sort of special relationship with an archmage. If both the Vampire Prince and an Archmage are backing the place, that is probably enough to get the others to leave it alone.
But the Technocracy might have their own reasons for wanting the place to exist. They want to exterminate the other supernaturals, especially the Tradition Mages (though some sourcebooks have suggested they have at least a working relationship with certain other groups such as the Camarilla….), so they would be unlikely to sponsor the place openly. But it would be a fantastic place to gather Intel. The Technocracy may well send agents there to overtly relax while covertly spying and may well attempt to bug much of the place. They will not openly declare protection, but the knowledge gathered may well justify subtly keeping the place safe. If you want to add an interesting layer of intrigue, they may well have subtly arranged to bring the place into existence…
Giving the wolves a good reason to support it is a bit harder, but the right relationships with a high Renown wolf can always be added in. Even without a reason to support it, the Garou probably have little reason to actively want to shut it down either.
As another, darker option there could be some entity with even more nefarious ends than the Technocracy and perhaps with more power than any individual technocracy mage who created the place and uses the changeling as a front. That power, perhaps a great but twisted spirit entity or other infernal entity such as those the K’llasshaa serve, might be siphoning away power from, and subtly corrupting the entities that frequent the place. It enforces a peace there so it can better use the place…unless and until that peace no longer serves it.
The Mortals Don’t Notice Because the Supernaturals are Even Less Conspicuous than Normal
As a general rule, the members of the oWoD have pretty strong reasons to avoid revealing themselves to the mortals. (There is a partial exception for the Mages who want to shift consensus reality, but even they would know that being too blatant would risk the Technocracy coming down on them and perhaps losing their willingness to tolerate the place, in addition to the usual paradox). This actually gets easier, not harder, when you gather a lot of them together. You will have plenty of beings around ready to help cover for any minor breach and blank out the memories of any mortal that happens to see things they shouldn’t have.
I am not aware of anything exactly like this in Canon, but the Midnight Circus comes close and serves as precedent.