The choice is most likely deliberate: this is a 9front crowd (Plan9's aesthetic choices are rather peculiar). It's what man(1) should display on a 80-column or so terminal.
Debatable: 80 columns ease reading, as each line is completely within the cone of vision. It's not enforced in their style(5)[0], but the code mostly and naturally (e.g. short variable names) falls within this limit (random sample[1]).
(I don't think that Plan9's "not thinking in terms of terminals", which is more of a "a better-than-X11 GUI is the default UI" is really relevant in this context)
EDIT: I don't have a Plan9 box around, but at least on plan9port, man(1) by default wraps lines to 72 columns or so.
Groff's HTML output is very poor. It's a sad day when it's more convenient to generate postscript and convert to PDF than it is to read the directly generated HTML output...