The number one problem with wikis and all other systems that try to 'store' employee's knowledge is that it requires people to make their knowledge explicit.
In your daily life and in your work, many things (if not almost everything) you do is based on implicit knowledge. You implicitly know how stuff is done, but to describe the steps you take and the thought processes you have, takes a lot of time and energy of your employees/colleagues. And then of course there's also the issue of keeping the knowledge up-to-date. Adding it is one thing, but keeping it fresh and ensuring people update the explicit knowledge in the wiki, also takes time and energy. Especially in IT, because the way things work changes relatively often.
And last but not least, people are often not willing to make their knowledge explicit, because their implicit knowledge makes them valuable as an employee. Overall you could say that the intrinsic motivation for people to make their knowledge explicit is very low.
Many scientific papers have been written on this subject. I suggest you try to find some answers there, although they may not be easy to find.