I just think the entire concept of desktop-targeted notifications is superfluous and thereby it doesn't surprise me that it is unpopular. Like, if I want a notification about something I probably want it to go to my phone; sure, it is maybe nice to also be able to browse my phone notifications on my desktop, but I don't need two actual "notifications", as it isn't like I don't have my phone near me 24/7: what I'd really want is the "notification" to happen on my phone and then let me remotely browse a synchronized list of notifications (preferably end-to-end encrypted to the running phone, not using a user) from my desktop... but registering for a notification only on my desktop--or even my laptop!--seems like an extremely niche value proposition.