I don't think there's a solo primary breeder for these guys, but several breeders scattered around the US are breeding the Blue Headed Pionus. Those are the most common of all the Pionus you'll see because they're the sweetest! My local shop, Sugarcreek Bird Farm in Bellbrook, OH breeds them too! You'll also sometimes see the White Capped Pionus and Bronze Winged Pionus but definitely not as often. Then you rarely see the other guys like the Dusky Pionus!
I'm not a breeder so take my opinion with a grain of salt obviously, but I've heard that they're not particularly terrible parents or hard to raise like some other species are, it's just a pain to try and match a pair that gets along (making you go through multiple females/males trying to find that match), and to actually maintain that pair that'll KEEP producing. I hear that many pairs will simply just stop producing. Pionus aren't known to be particularly hormonal after all, and hormones are the driving force of breeding! As you can imagine, buying several birds can quickly become quite the investment. So, less demand for them = Less investment into breeding them. You see even less of the other Pionus species for this same reason too, but also the other species tend to have less desirable personalities as well, being purchased mainly for appearance ( Less cuddly, more particular and nippy, etc. )
IMO you don't see them often in Aviculture just because they're overlooked. They're a small-medium sized, mild-mannered bird that isn't particularly flashy color-wise. They aren't known to have the exotic factors that'll draw non-bird people into having a bird. Macaws are large and flashy color-wise. Cockatoos are loud, entertaining, and known to be SUPER cuddly. Amazons are loud, entertaining, and talk. Greys are similar personality-wise to Pionus but are revered for the intelligence and talk factor. Usually, people don't get into Aviculture FROM Pionus, people don't typically have Pionus as an only bird, and usually people that have them have been interested in Aviculture for a minute. They're the sweetest and cutest little guys ever, but don't have those larger-than-life exotic traits of those high-demand species which unfortunately lead to them being overlooked.