That erasure would have been confined to one country, the destroyed one, had it not been apparently done in the flashiest way possible. Imagine a character that no one is supposed to know about, but they single handedly makes a move that not only causes big news, but the unknowing subjects of said figure actually rebel.
I mean the knowledge of Ohara was entirely cut down to two living people, and technically a third who escaped with the books but doesn't actually know the shit.
Meanwhile, the D clan is a shitload of people marked by name. The kingdom is just another pothole for Oda to drive around.