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.