[ ...Well, she would never be so bold as to ask that question outright, but Makoto had wondered about it, too. Are...the mermaids around now just really old? Or do they have another way to reproduce? Do they leave the kingdom to mate...?

...She is going to ask none of these questions. ]

I'm not sure about the "where", but "why" would be because the previous king persecuted the mermaids and treated them like second-class citizens. When they were fed up, they revolutionized.

[ As well they should have, in her opinion. ]