Bashfulness implies modesty and innocence. Modesty is virtuous and reflects well on their personality, and innocence is charming for several reasons. Most relevant here is that innocence implies (sexual) inexperience, which is innately appealing to males for evolutionary biology reasons, that is, as a male you can be more certain that the offspring you are raising are your own if your woman is not prone to having a lot of sex with others.
A possibly more relevant psychological factor that plays into it for me at least is that, with these girls' breasts being so unusually big, they are occasionally or even constantly forced to acknowledge the immense inherent sexual appeal of their own body, which clashes with their modest and innocent nature, causing them a strong sense of self-consciousness and awareness of her chest. They are unprepared for and can't escape from the kind of sexual attention that their body attracts. Especially when they end up dressing in a provocative manner like in many of these images, they are overwhelmed by the embarrassment in putting what they are most self-conscious of on display, but the girl does it anyway to catch that cute boy's attention or dress in the way she knows he likes, or maybe she has been coerced into it by her friends, or insert convuluted narrative here. Something like that.>>2488546