The problem is not the total time commitment, but the flexibility of when that time can be spent. The time expenditure for a written exam (writing the exam set, testing it, correcting submissions, having a grading meeting) can be spread over many weeks, with plenty of room to move things around to take care of other responsibilities, and easy tolerance for delays. Oral exams have to be scheduled in a dense block because students are only available during designated exam weeks, and any delays have cascading effects.