I believe that this is because twenty (?) questions need to use a tag before anyone (<20k rep) can suggest an edit to it, but this isn't mentioned anywhere on the Tag Info page. Can an explanation for the missing Edit Tag Wiki button be added to the Tag Info page? The current text is below.
There is no tag wiki for this tag … yet!
Tag wikis help introduce newcomers to the tag. They contain an overview of the topic defined by the tag, along with guidelines on its usage.
All registered users may propose new tag wikis. Click the button below to get started.
(Note that if you have less than 20000 reputation, your tag wiki will be peer reviewed before it is published.)