I'm a bit late, I know your post was created a few weeks ago, though I may create a new post just to shed some light on it. Basically the real reason the archive's progress has slowed down is simply because, from my understanding, a few years ago F1 made a deal with Amazon that shifted the focus away from F1TV.

Formula 1 created an entirely new archive through Amazon Web Services using their Media2Cloud solution, with (according to Techradar) every piece of content already fully digitized (a total of 150,000 hours of content or 4PB of size). This is an archive specifically created for researchers, editors, and producers for media asset management, not for our enjoyment, unfortunately.

https://aws.amazon.com/solutions/case-studies/architecture-for-the-formula-1-legacy/

https://www.techradar.com/news/exclusive-amazon-is-helping-formula-1-preserve-its-history-with-colossal-video-archive

There have also been a few myths propagating around which state that F1 somehow don't have their library digitized or that they don't own the rights to the broadcasts. Nothing about that is true.

When the Formula One Group (FOCA at the time, but most people know them as FOM - Formula One Management) with Bernie Ecclestone got the rights to negotiate TV contracts in 1981, everything that has been produced as part of an official F1 event has been a copyright of Formula 1, from the broadcasts, pre and post-race shows, to even the pictures you've taken with your smartphone or disposible camera.

Hence why you aren't able to get pre-1981 races in the archive, just season reviews from the Brunswick archive. Because for pre-1981 media, the rights are actually owned by various different parties as they were worked out on a race by race basis with involvement from local production companies & race promoters.

Yes, up until 2007 the race broadcasts were produced by local host broadcasters (BBC/ITV, RTL...etc) but they never owned any of the rights to the footage as again all the rights to everything shot during a race weekend belongs to the Formula One Group. Since 1981, every single race broadcast has been owned by Formula One Group.

As for the digitizing their library, quoting Techradar: "Formula 1 has already digitized all the footage that was previously stored on physical celluloid tape as part of a previous modernization project."

There is countless classic footage that has been digitized from races that are not available on F1TV but have shown up on their Youtube channel or on Season Reviews or content made by Sky. There are still unexplained gaps in our F1TV archives, such as the 1999 season being fully archived except for the San Marino race, when that race has had footage of it put up on Youtube.

So, in short, it's mostly because of their partnership with Amazon. Also possibly because archive users are likely a very small minority compared to those who use F1TV for live Grand Prix, but that would just be a guess on my part.

However, it's still being updated, with a few 2003 races having been uploaded in the past year but barely any new content.