As a generalization, one reason is because unlike many review sites or ranking lists, PG recommendations are not meant to be exhaustive, they are meant to be a selection of (somewhat) vetted, thooughtfully considered, recommendations, of a small subset of serious projects, that both exceed the minimum criteria for recommendation, and are best in class. And where there is no drastic difference, mainline/upstream well-established and organizationally mature projects are usually going to be preferred, since it’s less parties to trust, and usually that is where the lions share of the hard work and meaningful choices are being made.
PG recommendation sections preference quality over quantity.
why are there so few desktop Linux Recommendations.
I’m going to approach this answer differently from most others here, and just focus on the Linux distro ecosystem broadly. There just aren’t a lot of distros that matter that much, they exist to serve some niche, or to make some pain point simpler for beginners, or to appeal to young gamers seeking a particilar aesthetic and vibe, but they don’t meaningfully differ from their upstreams, and they don’t meaningfully impact the broader linux ecosytem.
This is why many/most longtime linux users tend to talk in generalizations about distros “debian based..” “RHEL clones” “Arch derivative” “Ubuntu based” etc. The vast majority of distros you’ve ever heard of are relatively light layers of changes on top of either: (1) Ubuntu (2) Debian (3) Arch (4) Fedora or to a lesser extent (5) OpenSUSE. There are dozens (maybe hundreds) of dependent derivative distros based on these, but with a few exceptions these are mostly somewhat small/modest layers of changes to the upstream and they are still very dependent on upstream for most of the distro.
(In my eyes,) The 3+1 major distro families in desktop Linux are (1) Debian (2) Red Hat/Fedora (3) Arch (+1) OpenSUSE. With few exceptions, almost any other desktop distro you’ve heard of fits within one of these distro families and shares most things in common with the upstream.
Beyond the big upstreams, there are usually at any given time maybe a half dozen or a dozen distros trying to do somehting innovative, cool, or meaningfully different. And many more distros that exist for a specific niche, need, or aesthetic, but aren’t broadly relevant beyond their niche.
Fwiw, unlike many in this thread, I don’t have a negative opinion of Ubuntu or Debian. And I think the stable vs less-stable release cycle conversation gets pretty exaggerated and over-emphasized. While I wouldn’t rank it ahead of any of the PG recommended distros (which are mostly closely aligned with my own pre-existing preferences) I would probably put Ubuntu on the shortlist, and I’d considered it appropriate for most privacy seeking people and most threat models.
Just because your distro of choice isn’t a PG recommendation does not necessarily mean it is the wrong choice for you or a bad choice in general. It just means it isn’t what (a small handful of people consider to be) the cream of the crop wrt to security and privacy.