Now, I'm biased: I find daemons more interesting, and demons to be the least interesting of those three. And I don't expect the three main fiend subtypes to have exactly the same number of monsters in them. But there is a slant towards chaos.I'm guessing this is a result of more Paizo staff preferring to write about demons. Which is fine.
While I do enjoy all types of outsiders (evil or not; I figure they all contribute flavour to a game, which is fantastic), I believe it makes perfect sense that the Abyss, being infinite, would spawn the most monsters. I would actually support Paizo in coming up with truly bizarre demons (not that we don't already have some of 'em—the Qlippoth are an especially nice take on that angle). I'd also like to see demonic versions of other outsiders (going a bit further than the useful, but necessarily limited, fiendish template).
I just love the idea of demons being easy—but very dangerous—to summon (and thus tempting for wizards and such), with the Abyss being a sentient plane trying to expand its influence and all (I like to imagine it actually ends up "swallowing" other planes, adding their corrupted, ravaged planar remains to its dark, chaotic, cosmic mass once they're completely overrun by demons).
It makes the multiverse feel like a fragile place somehow, that image of an infinitely deep gaping maw hanging just below it (to picture it that way). I like the idea of the Abyss being a primordial cancer on the multiverse.
But anyhow, yeah, it's all good! "An infinite horde of demons makes good sense" is what I'm saying basically, so all the better if the staff at Paizo enjoys writing about them! ;)