I saw a TV interview with the guy in charge of Dean's bother's search. They have been searching for his remains since October. It sounded to me that the Pentagon's POW/MIA office is researching these POWs on a case-by-case basis based on how good the information they have is regarding location, etc. They aren't just digging holes to see if they can find some remains, they are looking for specific people in specific locations.
It seems to me that the search for Dean's brother would have a high probability of success because they knew where he was being held the months before he was executed. Whereas trying to find the remains of a downed pilot would be very difficult.
From a related Reuters news story:
"Greer said 383 other Americans, including nine civilians, were listed as missing in Laos from that period. Some 1,875 Americans still are listed as missing from the Vietnam War in Vietnam, Laos, Cambodia and China, while 708 bodies have been recovered."