For starters, a ton of money to promote it and get airplay. Songs don't become hits if they don't get airplay. Or even moreso, video play. How many to 40 hits don't have a video to support it? Good looke help a lot, too. They'd have to have Brad Pit, or Morton Downey, Jr. lip-sync Buddy Holly's songs, if he was making videos today. Sometimes it's because the song is good, too.
Jerry