Where was the Predator all the while Dutch and crew were building the trap?
The build had to take a while. I find it hard to believe that the creature was gone for an hour or two without checking in on its game.
Throughout the movies we see the Predator do a number of things that are not strictly efficient in terms of killing its prey but seem more driven by honour. To give some examples:
Removal of the helmet and weaponry during the final showdown with Dutch (although there are some schools of thought that say this was due to damage to the helmet).
Use of wristblades to kill stunned or wounded enemies such as Dillon. A shot from the shoulder-mounted cannon would be far more efficient.
Not using their spacecraft for in-hunt transport or air support.
In addition, we know that the Predator is not simply trying to kill the prey as quickly as possible. It tends to pick them off one by one where possible to maximise terror and stops to take time-wasting and apparently unnecessary actions like flaying victims, stealing their bodies and removing their skulls.
Basically honour trumps efficiency for the Predator whenever it has the option. Arguably it follows such a code even when doing so greatly increases the chance of injury or death to itself.