1. Ban all your members. This is a huge undertaking (Usually), as most people have too many users to do this. This is an optional step; it would be best to just leave the people who want to be there be there. That way, when you make your site private, the regs can still do stuff. I'm going to assume that you know how to ban people, since it's pretty simple. Note: Before you ban anyone, make sure to give all the users notice that you are shutting the site down so they can cancel their membership. This way, the wiki you're shutting down will be removed from their "Wikis list."
2. Next, you'll want to make your site private. You can do this by clicking "Settings," "Permissions," "I Want to Change These Settings," and then under "Who can view this site?" (The very first thing on the page that comes up) click "Members Only." This means that only people who have previously joined the site can see it. Sign out and see!
3. Now is the final step to "deleting" your wiki. Before you do this step, really think about what you're doing, 'cause if you've banned everyone, you can't return. This is what you do: Go to your profile and click, next to the "Member Since:" thing by your profile pic, "Cancel Membership." You'll get a new page thing that asks you if you really want to cancel your membership. This is your last chance to rethink. If you click that button, boom! Your site is officially "deleted."

"Not to be a wise-ass or a smart-ass, but I have an answer to that question:
Simply because you no longer want that site to be up. It doesn't matter whether someone doesn't want you to delete the site or not. It's the Creator's or Administrator's decision to delete it.

Now since you brought it up, I'd like to add something to your post:
Anonymous, as wolfhunter said, there will be a user or two who might not want you to shut down the site. In this case, I don't think you would mind letting them be an Administrator, and let them take over the site. Now if you do this, don't expect to be promoted back if you cancel your membership and want to re-join. Yes, you made the site, but you left it. They don't have to make you more than a Registered User or Writer.
If you don't want anyone to run the site at all, then follow their instructions on how to 'delete' your site.

-Clay
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