The Arubans brought in a judge from outside of Aruba, BECAUSE their judge on Aruba hasn't the experience of this type of case to properly judge it.
Which is basically the same thing as moving a hearing or trial to a neighboring jurisdiction or calling in district attorneys from outside the jurisdiction for consultation or appointing a special prosecutor--something that is done quite often in the US. And that wasn't the only reason cited, either. Another reason was to avoid the appearance of partiality because of the suspect's father's affiliation w/ the Aruban courts. Bringing in an outside judge was a move to demonstrate impartiality.
All the more reason for letting the FBI assist in the investigation, to REMOVE doubts of impartiality. Thanks for helping to make my point! :)