Simple. People don’t believe He is coming back again because they don’t believe that He came the first time. Many people are quick to try and disprove Jesus’ existence and label Him as a mythical fable simply out of ignorance. The same skeptics who viciously attack people with Christian beliefs, are the same people who have never opened a Bible in their life, and if they have, they nit-pick at verses they want to read and twist them out of context to better suit their argument. I believe speaker Ravi Zacharias once said that truth in the hands of someone who hates truth, is only ammunition for them to be even more misguided (paraphrasing). It’s a sad truth that so many people are becoming atheists, especially in an age where we are entering the end times. It isn’t easy to convince someone that the Bible is true or that Jesus is real, but it is more possible when that person accepts their imperfections and accepts Jesus into their heart, and when they are open to listen and reading what the Bible says in its entirety. In terms of realizing that Christ is coming back simply has to do with how aware they are to their surroundings. The signs of His second coming are numerous, there are prophecies from the Old Testament that are coming true as we speak, yet people are blissfully ignorant to these warnings, so they aren’t taking God’s Word seriously enough. All we can do is lead as many people to Christ as we can before it’s too late. Hope this helped!