My thoughts on this are that a bp has to worry about a rodent prey's defenses. In this case the only form of real danger are the rodents teeth; striking the head and constricting the chest is more than enough to incapacitate its prey.

When you start talking about larger snakes you also start talking about larger and more dangerous prey. Utilizing every coil when it comes to larger prey could mean the difference between eating or not eating, and perhaps life and death.

I think its a matter of evolution, why waste energy on constriction when you don't have too.