The hill called Mount Zion today is not the hill the Bible originally meant by that name, a shift in meaning that happened gradually enough that no single decision explains it.
A name that moved
In the Hebrew Bible, Zion originally referred to the fortified hill associated with the City of David and later the Temple Mount itself. By the Byzantine period, Christian tradition had transferred the name to a separate hill just outside the modern Old City's walls, the location now conventionally called Mount Zion, a naming shift that occurred gradually through tradition rather than through any documented single event or ruling.
A monastery for Mary's final days
Adjacent to the site holding the Tomb of David and the Upper Room, the German Benedictine Dormition Abbey marks the traditional site of the Virgin Mary's death, or dormition, a separate but closely located tradition that adds a further distinct devotional layer to an already densely packed hill.
As this entry's companion article covers
The single building on the hill containing, on different floors, a traditional tomb of King David and a room associated with the Last Supper and Pentecost, used respectively by different religious traditions, is addressed directly in this entry's companion article rather than repeated here.