Having not sailed the F20c or any other curved foiled boat, I won't speak to why they placed the boards under the boat from a performance aspect.

I will speak of one good maintenance reason however. If you have a 10.5 ft wide boat sailing along at 25 kts in 20 kts of breeze up the Florida keys, hugging the coast as that is minimum distance, you likely can't see the water very clearly under the leeward hull. Now the keys have lots of these things called shoals, usually made of rock hard coral that will take your boat from 25 kts to 0 kts in about 3 seconds if you hit one. Imagine your curved foil hits one. Now imagine your curved foil sticking outside the boat hitting one. You now have a situation where:

a) you can't even see your foil moving in the water b/c its 11 feet away from you
b) your foil hits first as usual, but instead of a blunt impact, which is generally survivable, the entire boat is torquing around a 3 foot moment arm...snap is usually what happens when that kind of load is applied to a thin airfoiled surface.

With the foils under the boat, moments like that are a non-issue, its far far easier to know your full boat width so you can avoid hitting shoals, and more importantly your not worried about touching some F18's $1500 carbon daggerboard with your $1800 curved carbon daggerboard while on the start line. That would just get confusing and dangerous.