Because the NT isn't about YHWH, it's about who was sent to be the Savior. Christianity is about following the example of Christ and gaining knowledge in Christ, to the glory of YHWH. The whole Hebrew Bible is a build up to Christ, the Messiah.
Christ is King of the Kingdom. Christ is the mediator for all mankind. Christ is the High Priest and at Christ's name every knee will bend.
The apostles and disciples were followers of Christ , they were actual witnesses of Christ. So yeah, those of Christ were called Christians. Jehovah's Witnesses were standing on Egypt when the Red Sea was split . If you weren't there it ain't you.
Acts 4:10-12 NET
let it be known to all of you and to all the people of Israel that by the name of Jesus Christ the Nazarene whom you crucified, whom God raised from the dead, this man stands before you healthy.  This Jesus is the stone that was rejected by you, the builders, that has become the cornerstone.  And there is salvation in no one else, for there is no other name under heaven given among people by which we must be saved."
Anyone who claims to be a follower of the Bible should believe that is God's arrangement. He himself made those arrangements and that's how he wants things to be.
Now what I found interesting is when reading certain accounts, Christ says he has made God's name known. He also says it's the name that's been given to himself. John chapter 17 is mind altering. It's common practice for a father to give his first born son his same name. Junior. Hmmmm?
When I think about it, God never really told the Israelites his name.
"I am" is not a name, it's a state of being.