Set down in language that outlasts the saying. What is written can be checked; what was said cannot.
Old English writen, the past participle of the strong verb writan (to write); the irregular participle survives unchanged into modern English, where a regular verb would have taken -ed.
The one form a statement can take that survives long enough to be checked rather than only remembered afterward. What is written answers for itself in a way that what was merely spoken does not; there is something left to point at.