As far as I can see the reason for spike only is to get as many bulls (male calves) as possible to survive long enough to become big bulls.

They are only averaging 50% calf survival in many areas. So, 50% of the male calves are essentially dead before they even become spikes.

Then those male calves that are moderately successful become spikes and the highly succesful ones become raghorns or 2 points or whatever.

So by going spike only you are not allowing your most sucessful young bulls to be killed, then they can survive better,grow up and get more of your herd pregnant.

Spike bulls in general are not successful breeders so having your hunters take animals from that age class isn't going to hurt you as much.

When you start killing the prime breeders (raghorns and mature bulls) you're not getting as many cows pregnant as you probably could, even though the rut has mostly ended. So when you already don't have enough calves, you certainly don't want any more cows to not get bred.

Also, mature bulls will service the herd faster as soon as the cows come into heat,since they have harems. So that will in theory cause more calves to be born at the same time in the spring. The more calves that hit the ground at the same time, the better as predators can only get so many. If you left the breeding up to the teenagers, the calving season would probably be strung out longer and more calves would get eaten.

There really isn't anything they can do anymore until they get a handle on the predator problem and that isn't going to happen anytime soon.

The best thing they could do is shut down all elk hunting for about 10 years but no one would accept that and the Indians would still be slaughtering the big bulls.