Superfly got a very rough start back in the day when it turned out its implementation was missing a significant number of nodes that were key in many a popular Blender tutorial. This left a lot of people unsure how to work around the limitations of the Cycles implementation, and made the learning curve even steeper than it already was.

Some were able to make something work, but quite a few of those work-arounds were very complex and weren't easy to adapt by less knowledgeable users (this being the vast majority). Add to that a very sparse selection of built-in materials, something that over five years on has not meaningfully improved (why is there no default library of dozens of common materials?), and it's little surprise that Superfly became 'too much of a hassle' for many users. It didn't help that many prime content creators had by then started moving away from Poser, which meant that there was also no supply of examples to learn from.

PBR or texture based materials could in theory circumvent the imperfect Cycles implementation, but PBR requires a whole lot of new textures that many older Poser products simply don't have. That's where the lack of many new releases comes in again. Without a steady supply of new products, the Poser library of many users remained fixed in the Firefly era with no textures to support PBR renders and no Cycles materials to make Superfly seem worthwhile.

To this day the number of Superfly material packages is very limited. Poser itself still for some reason offers barely anything useful on this front.

It's a shame, too, because it is very possible to make Poser Superfly render great images. It's just a huge hassle to go spend hours reworking textures and materials rather than making actual scenes, which is what I suspect most users simply want to do.