Why does the Tag Wiki for the GIS StackExchange require users to have a reputation of 1500 in order to make edits? There are many tags that have no wiki entries, and lowering the reputation threshold would allow more users to contribute to those wikis. Any mistakes made in those entries could always be corrected by other users.
This is simply a carryover from a generic site template. It's 1,500 rep on Stack Overflow too, for example.
Remember it also requires being in the top 20 for that tag or a bronze tag badge. The badge is 100 upvotes on non-CW answers (questions don't count) and at least 20 non-CW answers posted. That's quite a lot for a single tag, and you're almost guaranteed to have 1,500 rep by the time you meet it.
It might not even be a good idea to quickly fill out tag wikis when a site is new and the exact use of certain tags is still being initially figured out by the community. The "obvious" tags should already be obvious and less obvious tags may be interpreted differently by various users, or change names.
That said, if you want to contribute to a tag wiki now, why not write up a draft and post it here on meta? Let other users contribute to (i.e. make it community-owned/wiki) and comment on that, and I bet a moderator would be happy to add it as an initial version in the tag wiki.