In my experience, documentation stored in source control alongside the code gets updated and maintained. Documentation stored in a wiki is left to rot.
For code documentation, that's definitely true, but you need places to document processes, designs, and other things like that.
Out of sight out of mind is one component of this. The other is how easily you find the documentation. Most people are willing to document if it's easy to do so, and most people will read the docs if they're easy to find.