We're not sure what you would consider to be a good explanation. We've heard it suggested that the brit milah on day eight speeds the woman's re-entry into the world of Jewish communal life and ritual, distance from which is connected to tum'ah. Without the brit milah, she is so absorbed in the physical world of being a mother (see Sforno) that she needs another seven days.
Please see our article Mikveh after Childbirth for an outline of the halacha as currently practiced. Nowadays, a woman needs to count seven clean days after the postpartum bleeding has stopped before she can immerse. Since postpartum bleeding generally lasts at least a few weeks (longer for many women), it would be impossible to get to the mikveh after only seven days and very unlikely after fourteen. Thus, there is usually no practical distinction between a girl and a boy.