Unlike in medical facilities, the radiation comes from multiple directions. Also, the crew moves around the plane. The vests would have to cover an impractically large area.

Also, when radiation is blocked by matter, secondary radiation is generated. Since the composition of cosmic radiation is for sure different than the one found in other occupational settings, lead vests might or might not have the same effect. Also, the aluminium skin of the aircraft already has a similar effect. There might not be that much to be done practically.

It would be interesting though to let flight crew wear radiation badges.