The reason for this is that coercion is (at least arguably) not an efficient way to elicit information that is actually accurate from people who are being questioned.
In the criminal justice environment, the most likely reason to coerce someone (other than through the physical coercion that may be needed to arrest them or otherwise control their physical actions) is to gain information from them. The police will, for example, want to get information from people regarding what they have done or what they know about the actions of others.
This is where coercion is arguably inefficient. First, coercion is, of course, illegal. But even supposing that a law enforcement official could get away with using coercion, it might not elicit accurate information. It is very possible that a person being questioned will simply tell the interrogator what they think the interrogator wants to hear. They will try to do this so that they can put an end to the situation in which they are experiencing coercion. This can lead law enforcement to waste time following up on false information.
For this reason, we can argue that coercion is not efficient in most criminal justice situations.