What an intriguing question!
As much as we would like everyone to be saved the truth of the matter is that not everyone will. However that is not the same as saying that Jesus’ death is not for everyone. Jesus death is sufficient and able to save the whole world but not everyone wants it. Lt me explain.
Christianity is about relationship with God or more accurately about a reconciled relationship with God. It presuposes that our relationship with God is fractured. This is what our sin does. It is rebellion against God. This may be an active attitude of disobedience or more likely a passive attitude of not caring about God. Either way God takes it seriously and our relationship with him is fractured - he is angry with us and will hold us to account for our rebellious attitude.
However because of his love for us Jesus died in our place and took the rap instead of us. As such he is willing and able to offer anyone forgiveness and reconciliation. Through him we can be reconciled with God. This offer is universal and available to all people.
Have a look at 2 Corinthians 5:16ff. Here Jesus’ death and resurrection is described as an act of reconciliation which is a relational term - “God was reconciling the world to himself in Christ, not counting men’s sins against them”. (2 Cor 5:19). Moreover it was a universal act for all people - the whole world. Rom 5:1-11 talks about Jesus death as being an act of love for us sinners so that we might be reconciled to him. Likewise in John 3:16 God gives his son that whoever believes in him shall have eternal life. That is this eternal life is available to everyone who believes in Jesus.
The thing to remember is that while this offer may be universal, not everyone is willing to accept it. God expects (reasonably I think) that those who accept his offer of reconciliation will to turn away from rebellion and live with Him as their God. That having seen the folly of their rebellion and sought God’s forgiveness, they would seek to be obedient to him and not rebel. Thus while the offer is universal, it comes with responsibility and not everyone wants this.
Have a look again at John 3:16-21. While God’s offer of salvation is for everyone, not everyone is willing to accept it. There are those who love to live in the darkness and who will not step into the light. But whoever is willing to step out of darkness and into the light and live that way will be saved. Also look at John 3:36. Those who believe Jesus and accept him will be saved but those who reject him will remain under God’s judgement. After all why should God forgive those who want to continue to rebel against him?
I hope this helps you. If you do want to talk further please reply with other questions.