The accepted answer neglects an important feature of community-wiki questions: they do not confer reputation on the asker. This is an important feature, because some users do not want to gain lots of reputation for questions which tend to be high-traffic and highly upvoted but (often) convey less subject-area acumen.
On many other sites, questions are routinely made CW. For the first several years of SE's existence, on SE sites one had the option of making a question CW upon asking it. Many longtime SE users (like me) view the removal of this feature as slightly obnoxious. It was slightly obnoxious provided that requests to convert questions to CW were routinely granted. If they are not being granted, then I at least view the change as a very obnoxious loss of functionality. For a platform whose motto is "We don't run XXXX, you do!", SE has been slowly but steadily moving towards a model which micro-manages user contributions. I would welcome moderators who push back against this a bit, as do most or all of the moderators on the other SE sites I frequent.
Added: There are further nuances of CW which are not discussed in the accepted answer. A non-CW question is attached to a single user. Although high rep users can edit the question, in the culture of many sites -- including this one -- edits to questions are done sparingly, mostly at the level of copyediting, adding links and removing obviously problematic content. There is the sense that a question is still being asked by a specific person and that one should not mess with it too much without their consent. Making a question CW is a clear signal that everyone is encouraged to edit it as much as possible. Losing this feature is...is a loss. I can't understand why that would not be desirable on a site like this.