When I was a scientist working with the (British) police, one thing that
stuck me was that uniformed police have as little as possible among their
equipment which can be taken away and used as a weapon against them. When
we wanted to give them hand-held computers for a project, we had to find
hand-held computers which didn't hurt you if someone beat you over the head
with them.
A Troll carrying an iron weapon is bringing to the fight a weapon which
could be turned against him and his mates to horrid effect.
There are other reasons why trolls wouldn't use iron, but that one seems
enough on its own.
Andrew