User blog comment:Kirkburn/Technical Update: September 2, 2015/@comment-3388044-20150904013930/@comment-3388044-20150904172135

I think the reason we cannot edit the JS sub-pages in our personal userspace is a technological limitation: The techies are releasing the locks on JS pages title by title, regardless of namespace. And, without a list specifying all such pages across Wikia, they cannot be unlocked using the current method of doing so. That is, the locks weren’t taken off of JS in userspace, per se, they were taken off of four specifically named pages only: global.js, common.js, wikia.js, and monobook.js.

Thus, for JohnDoe123 to be able to edit his  page, that specific FULLPAGENAME would have to be whitelisted. And, the same goes for each of his other JS sub-pages. And, the same goes for all of JaneDoe123’s JS sub-pages (they just got married!). Etc. Etc. And, that’s a heck of a lot of work for techies who are slammed for time building a JS review system, among myriad other new security features. So, we get the top-level JS pages only, and that’s that.

Mind you, this is just a guess … :S