I've pondered questions like this for years, and I don't honestly know all the reasons why some people decide to believe in Christ but others reject Him. It even happened to Jesus; on one occasion, the Bible says, "many of his disciples turned back and no longer followed him" (John 6:66).
What is clear, however, is that when someone deliberately turns their back on God and chooses to live without Him, they alone are responsible for that decision. They may blame others; they may blame their parents; they may blame the hypocrisy of Christians they've known, but ultimately they can only blame themselves for rejecting God's love and salvation. The Bible warns, "The one who sins is the one who will die" (Ezekiel 18:20).
The Bible also makes it clear that sin has consequences, and we can't escape them, either in this life or the life to come. We may convince ourselves that we don't need God and we'll always be happy without Him, but eventually we'll pay the price in heartache, insecurity and judgment.
But the Bible makes one other truth very clear: God still loves us, and He wants to forgive us and put our feet on the right path, if we'll only repent of our sins and open our hearts to Christ. Pray for your brother, that he'll realize the folly of his ways, and realize, too, that Jesus died and rose again for him just as much as He did for you.