It's usually attributed to a few reasons. No time, no experience (this is usually high on that list) or fear of making mistakes. That last one, however, is the funny part because with wiki's, the information is editable and most of the heavy lifting is done using templates I created to lessen the workload a TON! I'm sure there are various other reason why people can't or won't help, but I'm very grateful to those that do help because finishing the wiki on my own, while possible, would take up too much of my time. I barely have any nowadays as it is! Sure, it'll clear up soon, but this way, the wiki won't take forever and a day to get filled.
I know my current contributors will do a fine job. They seem capable and eager to help, so that is always a plus.