At the foot of the cross stood three women, Mary the mother of Jesus was among them. A fourth person, John the beloved disciple came back. On the Cross, Jesus gave His mother into John’s care. John, one of the twelve, knew what happened to Mary. I would believe he told the others, if the others were not there when Mary’s demise occurred. Why didn’t the apostles write about it. Well, one, Mary’s demise may have occurred after the other apostles (at least some or all of them) had themselves passed from the scene. Remember, John was the one apostle who was not to be martyred and outlived them all. A second possibility is that given the proliferation of Gnostic heresies that beset the early Church, the church fathers felt it might impact heretical ways of thinking. And remember, the New Testament is about Jesus only. It isn’t a history book to tell us about how Joseph lived and died, how Mary died, etc. Those things are irrelevant to Sacred Scripture. True, belief in the Assumption goes back to the early Church fathers; they probably just didn’t think the Assumption was anything that added to the story of Jesus’ saving ministry.
And an anecdotal supporting logic I have come across is that the early Christians were absolutely manic about collecting the relics of those who died for the faith or were important to the story of Jesus. Yet there isn’t a known relic of Mary to be found anywhere. At least not that the church has authenticated. Maybe because there was nothing to collect because Mary (or at least her body) was nowhere to be found???