Um, yea, that's what my first thought was. I did a quick search and found this interesting examination:

http://mot.vuse.vanderbilt.edu/mse150/Fracture/Ductile/ductile.htm

1018's only real merit (as I recall) is that it is easily machined. The list of advantages seems to end there in my memory.

Ductile fracture is, in part, due to work hardening. A steel that easily work hardens wouldn't be my choice for a hitch pin, especially an adaptor pin like this critter.