One of the killers for any sort of project that you do in your own time is the lack of a deadline. If there is no time limit for when something has to be done, you're relying on yourself to get it done in a reasonable time.
For some people this isn't a problem. They're self-disciplined enough to get the work done in a reasonable time. But for the rest of us, we're not so lucky :)
Challenges like Round of Words in 80 Days (ROW80) are fantastic for helping to set goals with a time limit.