Why are PP and L5 mentioned so often? They are available as new devices and that apparently has some value of its own in our society. They also have "unique" hardware features of arguably questionable usability (hardware kill switches), and are being used by many mobile Linux developers. That's why they are popular. It's not a conspiracy.
Regarding the Gemini and the Cosmo: While nice products in principle, they by design only attract a niche of users that want this form factor. While they in theory have Linux ports, these (except for the Sailfish X port for the Gemini) are unmaintained. Debian 9 "Stretch" is just not enough these days. Therefore and because the main Android OS also feels rather abandoned, there's not much interest in these devices. I would love to use them more, but there's just no interesting software and I don't have the time and resources currently to get that Debian port back to an acceptable state.
With regards to postmarketOS or Ubuntu Touch devices, especially those that also run postmarketOS stable (Community devices) I wholeheartedly agree. They do deserve more attention. These are really nice devices that are well supported and cheaply available.
But, sadly, having been a content creator in that field for a while I can tell you: Videos about these devices get way less clicks. This might be the self-amplifying effect that leads to less interest into these devices. It's sad, and if I ever make another video, I'll make it about a non-PINE, non-Librem phone only to then be asked in the comments why I don't share an update on the PinePhone or Librem 5 instead.