Society spokesman Peter Fricker says animals should not have to suffer and die "just to entertain a crowd."
The animal could not get up after having its head and neck twisted to force it to the ground as part of the competition.
Three veterinarians in the arena made the decision to euthanize the animal.
Stampede officials and independent livestock specialists say they will review the event video to determine any contributing factors.
They point out that recent rule changes prohibit competitors from continuing to wrestle a steer that has splayed its legs and cannot fall safely.
Stampede officials says that does not appear to have been the case in this situation.
They also say the death of a steer during the event is a rare occurrence, with only one other steer fatality in the last decade.
Fricker said the rules changes obviously have not worked.
He said the event should be suspended and reviewed by an independent panel of experts to determine if it could be made humane and safe.
"If these events are so hard on the animals that it can kill them, it's obvious they are suffering pain and stress," he said, adding that steer wrestling has no historical connection to ranching and was invented for rodeos.
"It has nothing to do with agricultural heritage," said Fricker. "It's just a cruel and dangerous circus act."
Concerns over animal welfare have grown in recent years and were magnified in 2010 after six horses died at the Stampede.
In 2011, the Stampede adopted new rules for rodeo events and chuckwagon races to try to provide more protection for both human and animal participants.
(CHQR, The Canadian Press)