The tag wiki suggestions were declined because the tag was used just from a question; in that case, the tag is automatically deleted after X days. In fact, when I used a custom declining reason, I wrote:
The tag is used by only a question; such tags are automatically removed. It's better to provide a tag description for tags that are going to be kept.
When a tag is removed, its tag wiki is not anymore accessible, if not to moderators, who see the list of orphaned wikis as in the following screenshot.
As you see, it's not possible to understand to which tags those tags wiki are referring, and Stack Exchange is not able to re-associate a tag wiki with its tag, in the case there are questions re-using that tag.
Recently, I approved a tag wiki for a tag used from just a question, but the reason I approved it is that I misunderstood its purpose: While I thought it was related to a Drupal 8 concept, the tag is really about a third-party module. If I understood that, I would not have approved the tag wiki, simply because the tag is used from a single question.
About the requests to add new tags, those should be created if there is already the need of using them; they should not be created because a user wants to use it in a question.
The criterion I normally use to understand if the tag is necessary is: Is there an expert behind that tag? For example, to answer questions tagged views, you need to know the Views module; to answer form-api questions, you need to know the form API; there isn't an expert that answers to error questions, which would be any question about code showing an error. In the latter case, the tag is too broad to be helpful.
Clearly, that is not the only criterion that could be applied, but it helps to trim out the possible candidates as tags because not every word that appears in a question needs to be a tag.