I stumbled across this old news article that espouses a more specific timetable for the lack of online seat assignments.

"The airline will be temporarily restricting the time passengers are allowed to assign their seats online."

"But from now until November, both(sic) all passengers would only be able to assign their seats between 48 hours and one hour prior to departure."

"A source at Asiana explains that the change is needed to accommodate the introduction of a new online check-in system"

Asiana temporarily restricts pre-seat assignment time