I think it’s because of the nature of the subject matter. F5, F7 and F9 all have substantial marks available for calculations. Calculations are particularly easy to write effective MCQs for.
F8 has no substantial calculations. Producing good MCQs (questions that are not ambiguous and in which all the potential answers are attractive) is much more difficult. Also, ACCA want to see how you respond to scenarios and, really, you have to write that out in essay form.
Having said that F4 (law) is all MCQ, so the auditing exam might develop over time.