One simple reason is that we tend to think that God loves us for what we do. If God loves us because we’re nice, it becomes easy to look down on mean people. If God loves us because we’re generous, we have no trouble being critical of stingy people. If God loves us because we’re liberal, open-minded people, we tend to look down on bigoted, conservative people. If God loves us because we’re traditional people of faith, it only makes sense to believe that God loves progressives and atheists less than He loves us.
The Bible teaches that God loves us NOT for what we do, but because of WHO WE ARE. We are His creation, the focus of His affection. Jesus died as a demonstration of God’s love for us…not because we’re good, but because we need to know His love. That’s grace: God loves you even when you don’t love Him. When people understand grace, they stop thinking of others as less worthy of God’s love.
When we see others through the lens of good works, we tend to be critical and exclude others. When we see others through the lens of grace, we tend to be generous and compassionate and include others. When we measure the effectiveness of the American church at reaching out to the unsaved, it seems obvious to me that we seldom offer to others the same grace we’ve been given.