A lost writing system becomes crackable when the same text turns up beside a version in a language people can already read. The known version acts as a crib, letting you test guesses about the unknown signs against a meaning you already have. Without such a parallel text, the signs on every temple stayed unreadable for centuries.
When a people's writing can no longer be read, their whole world turns into mystery, however mighty they once were. Their monuments survive, but what they thought and recorded does not. Recovering the script throws open a door to everything the writing preserved.
When rival powers are at war, solving a famous puzzle becomes a prize for the nation as well as for the person who solves it. This raises the stakes and turns scholarship into a competition between rivals. Personal glory and national pride push the search forward together.