I don't know all the reasons. I know a few.
They think the church is no better than the world. They leave people to hurt. They talk about love and forgiveness but shake their finger at the wounded and the broken. They talk about grace and forgiveness but leave the broken and wounded to die in darkness.
And in that darkness people need someone to be Jesus to them in a real way.
I know a pastor who signs his correspondence "until all hear" but it's not enough to let people hear about Jesus we need to love them like Jesus.
It is not the healthy who need a doctor Jesus said. There are moments in our lives when we are healthy and their are moments when we are sick. Even as Christians. And it's sad to see Christians forget to help those inside the church walls find healing for their sick and brokenness . We communicate in texts instead if taking the time to make a phone call.
People are always shocked when a Christian commits suicide. Didn't they know Jesus? I bet they did. But maybe the pain of the stones being thrown at them was too great. Maybe when the darkness closed in, no one was the flashlight of grace and so the darkness just enveloped them. Maybe when they were hurting instead of a hug they watched someone look at them in sadness.
As humans we are quick to judge not hearing the whole story or even thinking about how the other person is feeling because its easier to deal with their sin instead if our own. It's easier to eliminate "those sinners" from our lives instead of loving them right where they are at. But I'd say that if we don't figure out a way to love our own to give grace, we might be digging the grave for very people we say that we love by leaving them in their loneliness and hurt.
I don't pretend to have all the answers but I do know one - love like Jesus always - everyday no matter what.