In the same manner in which a released hostage has to be debriefed, you may need some time to adjust to the bright light of the cause and effect timeline. It was described as a feedback loop. When you look into a series of mirrors, you will see an infinite number of reflections.
So, if Abe told Aaron about the box and it turned out to be a bad decision, he could go back and change that event. This new cause will produce a new effect as, Aaron’s response to not being told about the boxes. Aaron may attempt to steal them or use them in an unsafe manner. If Abe goes back and tries to relive the first timeline, he may unwittingly be starting a loop where he continually tries to change that event. Even if Abe has a third or fourth option, it just complicates matters if the final option leads back to reliving the first option.
But, you may say that Abe would be aware of the infinite loop. Aaron would not be aware of it. Remember though, that very quickly, both Abe and Aaron are using the boxes. Thus, neither one is aware of what the other is doing. This allows both of them to be in a continual loop without recognizing that fact. There are much more than one or two events in Primer. Keep your sunglasses on.
Both Aaron and Abe are controlling time. If everything was discussed openly and honestly, there would be no problems. Every time one of them uses the box, they erase the previous timeline from the others memory. They need to trust one another. It is hard to prevent the other person from using the box. So they continue to deceive one another, pretending to be their past self. Abe doesn’t realize at first that when he exits his fail-safe, Aaron(2) is already in the timeline.
