Why are most vintage pen nibs fine and firm?
Nowadays most fountain pens are sold as special-purpose instruments, used for signatures and personal notes, but seldom for extensive day-to-day writing. Bold-writing pens are thus particularly prized.
In times past, however, a fountain pen was an everyday tool, and one's primary writing instrument. And functionally, a fine nib offered significant advantages. A fine nib permits more writing between refills, while a fine line dries more quickly, reducing the risk of smearing.
Early fountain pens typically have quite flexible nibs, but with the widespread adoption of carbon paper in the early '20s, nibs stiff enough to make carbon copies ("manifolding") became the rule at least in the USA. Firm nibs are also more resistant to being deformed by excessive pressure ("sprung"), which is why very few modern fountain pens are currently sold with flexible nibs.
Nib preferences varied from place to place as well as from era to era. British pens, for example, are much more often found with broad nibs than are American pens. Parker's standard US mix at the end of the 1920s is cited on page 127 of Shepherd and Zazove's Parker Duofold: "the normal directive for nib points was 80% rigid and 20% flexible . . . the normal distribution was 44% fine, 44% medium, 10% extra fine, 1% broad and 1% stub. A specific order was required for the oblique point."