Because in general we tend to be OK with wearing helmets for sports when doing so seems a reasonable tradeoff but not for day to day activities. Clearly helmets would be beneficial for certain types of injuries but it's hard to imagine many people willing to make the tradeoff. (I would also wonder if they'd actually be a net benefit given potential reductions in visibility, etc.)
Cycling (in the US) sort of falls between those two categories but, at least for adults, many who use bicycles as transportation also use them as a sport.
Cycling (in the US) sort of falls between those two categories but, at least for adults, many who use bicycles as transportation also use them as a sport.