Probably because a limited offer is more enticing. If you can always put it off until tomorrow because it's always free, you might never bother, but because your time is short you jump on board and the game will do the rest.
"There are many parts of my youth that I'm not proud of. There were loose threads - untidy parts of me that I would like to remove. But when I pulled on one of those threads, it unraveled the tapestry of my life."