I dunno. Probably a war and someone wanted ta' get rid of unsavory things in the past. Stuff like that happens sometimes, though not for a whole century.

[Least not where he's from, but since she's posed the question, Heisuke tries to think of a reason. Why would an entire century just disappear? And a history written in pictures? Oh! OH HE'S GOT IT!]

I GOT IT! Maybe someone made a huge blanket!!