How did the rounded-triangle pick originate?

The Middle Eastern risha/mizrap is way more comfortable to hold, and players can do the same sort of thing with it. Western lutenists were using risha-style plectrums in the Middle Ages and every other string plucker was using their fingers until the 18th century (when mandolinists started using something different).

Indian string pluckers use a variant of the triangular pick, but being a larger object made of coconut shell it isn't as hard to hold.

Continuously holding something with a tight grip isn't good for your blood pressure.