From what I've seen historically, the engine configuration had to do
with 5 factors:


1. how to machine it, feed it, and get it lit (this was the major
issue before WW 1)
2. how to keep it cool (hence the popularity of the radial, which was
originally designed to power the Langley aerodrome)
3. because everybody else did it that way
4. cost
5. how to cram it into the airframe

Number 3 now seems to be the most popular reason to use an air-cooled
flat.

Harry