Why 予期せぬ is more common than 予期しない?

Question

予期{よき}する (to expect) is a nominal verb with two negative forms: 予期せぬ and 予期しない. Why the せぬ form is more common than the more familiar しない?

Answer 1

That's simply because It's a fixed expression originated from classical Japanese.

We have a number of such expressions. See: Nuance and conversational use of なきにしもあらず