There are a lot of tags where a tag wiki wouldn't fit or is just plain stupid.
For example... For the tag programming-languages a wiki could be made but would it be useful?
Why not let it just evolve?
If you think a tag needs a wiki entry create it and just ignore the ones you think are OK with out one.
Others will create other tag wiki's as they see fit.
Eventually we'll have wiki's for a set of tags that at least one person has thought would be useful. The ones that don't have wiki's will be the ones that everybody has thought wouldn't be useful.