Not sure why on F-class, but it is the same in most other precision shooting competitive events. Stainless resists corrosion, machines easier because it is a little softer. The softer stainless is arguably easier to make more "precise" during the manufacturing process and usually will give better results with a quality hand lapping job. Plus stainless looks better
However, Chrome Moly disipates heat better and may give a little better barrel life due to extra hardness. If made properly, a good chrome moly barrel can be every bit as accurate as a stainless barrel.
I suppose it's preference. But some of the top barrel manufacturers don't even offer chrome moly so that may be telling you something about what the pros think makes the most accurate barrel...