In general, and on just about any site, the answer is usually the same: because it's not possible to maintain multiple versions of the code! For us in particular, something like site skins are easy to have multiples of (because of the way the templating of the site works, we don't have to change every file for a new site skin, just one file that is then called everywhere else), and it's generally pretty easy to code an option to "turn off" particular things that can have an on or an off switch (although whether or not there's enough demand to make that option, when more options are generally bad UI, is another story), but when it comes to single pages, like the update page, maintaining multiple versions is a waste of developer time and effort. For instance, let's say (theoretically) that there are two versions of the update page, and someone discovers a major bug that exists in both of them, and that bug needs 20 hours of developer time to fix -- if there are two versions, that's 40 hours of developer time right there, and the other 20 hours could be used to do other stuff. Or, let's say there's a new feature being added, like a new commenting security option, and it takes 20 hours to add it in -- now it's either "only add it to the new version and screw over the people who are using the old version", or "spend double the time updating both the old and new versions".

So, when there's a "legacy version" of something, it's really tempting to just leave it alone and not make any updates -- no new features, no bugfixes, no nothing -- and it just sits there and bit-rots until it's utterly useless! So it really doesn't benefit anybody to maintain the old version alongside the new version, once you're satisfied that the new version isn't lacking any functionality that the old version had and that the new version doesn't break too badly for any of your users. Which is why we were careful to run the prototype by y'all a few times until we were satisfied that we hadn't made any major usability or accessibility mistakes!