Well, look at the ranking Chibs just posted. "KonKon wasn't that popular" and "Eri wasn't popular until platinum" don't exactly seem to resonate with those lol.
As for photobooks, they are a good indication of if a girl is getting pushed by management.. while sales (of the official store as much as of PBs..) are a good indicator of wota popularity. For Miya a lot of us got the feeling it's her decision. As for Risako yeah she got 4 and then no more, still 4 is a good number compared with most Berryz members. Like Sohee said there can be variety of reason. Momo is the only one who got more than that, but then again Momo is the most popular Berryz member so it's to be expected.
See it like that.. Sayu was the girl 6th gen girl (outside of Miki but let's ignore Miki for this exercise) to get a PB, probably because they figured "Well she can't sing, and she sort of can dance but nothing special, and her personality is a copy of Rika's, but she's pretty, so let's give her a photobook". Then, it probably didn't sell, because she wasn't very popular (I think? I don't have numbers so this is a guess). Cue management going "fuck this shit" and Sayu getting no more PBs for three year. Compare Eri, who got her first PB a few months later, and then kept getting them every year, probably because, just like Reina's, they were selling pretty well. That's my guess. It took longer for Sayu to be popular.. for obvious reasons.