I think there were a few reasons, if you are making your own in house Fasteners
, why not reduce the head size to save space and weight and materials.
The better quality marques like Ariel made a point of having particularly well made Fasteners
If you are starting with hex bar a reduced head Across the flats size saves a lot of machining, also allows for neater construction, many of the clearances around fastener heads are tight on a motorcycle, designers made stuff as tight as possible to look pleasing ,and save weight/ materials.
Once you have done all this you might as well add a dome/ chamfer, thats what separates machinists from the animals.