Why would non-believers want to be in heaven?
Instead of asking how can a loving God send people to hell, the first question ought to be why people who don’t believe and love God would want be in heaven. If you reject the rule of God in your life on earth, you would hate the rule of God in heaven for eternity. Furthermore, if you don’t like Christians talking about Jesus in this life, why would you expect to enjoy hanging out with Christians for all time? My gut feel is that when people ask this question they have a bit of a wacky view of heaven, thinking that it is really a long, white, sandy beach with crystal clear aquamarine water lapping at its edge. That sounds nice but there is a strange absence of Caribbean beach resort language in the New Testament. Heaven is primarily about a relationship with God and Jesus and other Christians rather than a really nice place to have a holiday. When an unbeliever understands this, heaven seems like a less attractive place. An in a sense, God honours the wishes of non-believers; if they are disinterested in relationship with God in earthly life, he will not force that upon them for eternity.
Why would God allow anyone to be in heaven?
The more pressing question is why God would allow anyone to be in heaven. The testimony of the Scripture is that everyone turns away from God and that no one is naturally inclined towards God by themselves. This is what the Bible calls sin, and hell, spiritual death, eternal punishment are the just and right consequences of sin. On this understanding God would not want anyone in heaven. But the greater truth is that God sent his own Son Jesus into the world to save people from their sins, from hell and from eternal punishment, should they put their trust in Jesus. Nobody deserves to be with God in heaven, it is the gift he gives to people, on the basis of Jesus’ sacrificial death in the place of people who put their trust in him.
God’s justice demands that sinners be punished with hell; his love means that he won’t force relationship with God in heaven upon people who have opted against it with their lives. But his great grace and mercy means that the possibility of eternal life with God and Jesus in heaven is open to those who turn back to God and trust in the death of Jesus.