I think this is the biggest factor. I think were it our LE, that soemone might have spoken with the security guards, but doubtful that they would have been arrested. Meanwhile, I do think the 3 guys would have been taken in for questioning and their car looked at, right off the bat.
I'm not sure just when they were first spoken to, though so it may have already been too late for evidence to have been apparent.
The thought occured to me today: Perhaps the brothers really don't know what happened. They may have just driven J and N to the beach and dropped them off. They may have felt pressured to lie to protect J and possibly PVDS because of PVDS's power and connections. And they may feel they need to leave Aruba because they changed their story and were no longer willing to stick to the story J and PVDS asked them to tell. Thus, they may no longer be safe on Aruba. Perhaps their statements can be used in court, even if they themselves are not present.
Just a theory.
Natalee's family were the ones who found out about Joran and the Kalpoe brothers. They found out within a very few hours after landing on Aruba, and they called the police's attention to them. After that, the police totally botched everything. The Aruban police did not have the Joran-Kalpoe lead until Nat's family gave it to them. That says a whole lot for what the true situation has been down there.