Why have they not believed? Romans 10

Why have they not believed? I can be so certain that the Bible is true and Jesus rose from the dead, but why do so many people not believe it? Maybe it's because the message isn't clear, the Bible is hard to understand sometimes. Or maybe they've just never heard the good news. For many of us, this topic is close to our hearts, we have friends and family that don't believe in Jesus and we don't know what to do about it.