Board Thread:General Discussion/@comment-15305799-20140308040344/@comment-4018164-20140310165151

The protection of user pages is to prevent scenarios on which many users don't contribute and improve the wiki, yet keep adding further content to theri user page. It's for the base user page only, but if it's based on some content on a different page, that page could be locked as well. I'm not sure about what limit you're asking about, but generally this is applied on everyone who has over 100 edits in the wiki (there's no point in applying it, of course, on users in groups like Staff OR VSTF).

And that's right- the more you edit other namespaces, the lower your user namespace editing ratio is, and the more you can edit your user page.