Before 20, I imagine it's just a matter of the stars aligning on the parents agreeing, the idol agreeing, and the agency having enough willingness to put the resources into it to ask when the idol hasn't already made it known that she wants one. After 20, they usually get one if they really want one. As many questions as I have about the decisions sometimes made by the office, I do have enough faith in them to not push too much when a girl really doesn't want, isn't capable, or is unprepared to do something. In the case of Maa-chan and Kanatomo, they have been obstinate in their refusal. Kntm has said it's because she's insecure about her navel and Maa-chan has also mentioned her own insecurities in the past about being in pictures. I expect that the office tries to strike an authoritative balance between helping the girls overcome some of those insecurities while avoiding being outright authoritarian with their pressure.
To that point, although Uemuu did mention that she didn't like swimsuits (putting them on, I believe was the actual complaint, though I could be misremembering), I don't think the agency would have really pushed back if she had just outright refused to do some of the shots when she was given more bikinis than she asked for in her first photobook. They have shown that they value the input of the girls when they have requests or want to do a non-standard photobook. Whether or not those requests are always the desires of the girls themselves or a concession to their parents even after 20 can remain unknown, but those requests are usually respected.
For Yokoyan, she probably asked for one, the parents probably agreed, and she's adorable and popular enough that the agency probably went with it immediately.
. I'm so happy they are actually pushing her. 
