I wouldn't say it's by design - more of a fun consequence of the way the BBC's web platforms have been retired so far. Most of the retired platforms generated static content (some XML, some HTML) so when the platform needed to be switched off it was relatively trivial to archive all of the content. In about 2013 the website was transferred piece by piece to a dynamic platform, which it (mostly) still runs on.

The current platform is already slated for decommission some time in the next couple of years and there's still no cohesive plan for how pages on that platform will be archived. My guess is that they will be rendered on the new platform(s) indefinitely, since everything is driven by a CMS API now. I think it's kind of a shame - there's something nice about being able to look back in time without the pages being distorted through the lens of the wayback machine.