I don't think it's necessarily luck, there can be plenty of factors like if you get along with someone they'll probably be a great player. Also if they're a new player I think how you introduce the game and "rules" aka how to play. I always let new players know that it's us v.s. the DM. If...
Just to answer the initial question, I think that many campaigns don't ever finish due to the fact that people have busy schedules. More players = more conflicts. Also! I personally don't want my campaigns to end, I'm completely happy setting a game down and picking it back up when at least part...
One thing about fairies, instead of trying to kill you or eat you they'll try to rob you but in a creepy sinister way. Like your name or your memories.