The same thing stopping Micic or Cuba from sending a full team the last several years. Many countries only offer the Olympic weights or will only offer 'coaching support' if they enter in Olympic weights.
What that might mean is that 'yes' they can go 61 KG or 70 KG (like Gomez), but they will absolutely need to finance (or have their team) finance their trip.
For RBY, it could simply be that he knows 57 KG is his Olympic weight and he knows he needs to get more reps weighing in (or around) 57 KG. Gomez doesn't seem to have issue with making 65 KG and his style allows him to be dangerous at 70 KG, still. Also, 65 KG -> 70 KG is much different than 57 KG -> 61 KG. I know it is actually a lower number difference, but I feel like you see more 65 KG guys up at 70 KG than we do 57 kG guys at 61 KG, which means your opponent's styles will be the same.
In summary, to answer your question: I don't have a damned clue.