The flathead screwdriver type you're referring to is usually referred to as a keystone tip, at least in the USA. There are other flathead/slotted screwdriver types that don't use a keystone shaped taper design. The main style that doesn't use the tapered design is called a cabinet tip. Cabinet tip screwdrivers usually have a round shank with the tip bevel ground to a slot tip. Some gunsmithing screwdrivers also lack the taper, however they're more likely to be hollow ground than bevel ground. As far as slotted screwdrivers go there are actually more designs and geometry differences than a person might think of. The design you're referring to might simply have been a lower cost to manufacture since after hammering the tip into the tapered shape there would be less grinding to finish it. Alternately there might be some design purpose that isn't generally known, such as the ability to ream small holes, similar to the tapered angular design on many combination pliers, the head of which can be used for reaming conduit.
