Cylinder Heads - Valves, Shape, Material and Finish
For the very good and simple reason it minimised airflow obstruction by having the least amount of material in the way. Very plausible, and a design that many have stayed with to this day - surprising considering the magnitude of expertise housed in the more recognised A-series tuners. Probably something along the lines of 'if it ain't broke, don't fix it', or perhaps (and more likely?) 'Can't teach an old dog new tricks'.
However, as time and technology progressed, man's ever-inquisitive nature started to determine more exactly what and why - and airflow activity was not passed over. Not only 'how much' was being measured, but 'where and how' to further determine where improvements could be made. After all - it has long since been realised that the engine is essentially an air pump. The more you get in, the more you get out, so improving the air's tortuous route to the cylinder has to help.