I have never flown over the Hudson myself. However, since it is a river, it extremely easy to see its edges. With VFR corridors over land, pilots may be less certain of the edges and stray more easily into each others way.
In terms of flying in controlled airspace over the Hudson, ATC may be too busy at times to permit it, especially for sightseeing.
Is there more about the plane reporting engine trouble? Was it losing altitude at the time of the collision? It may have been impossible to see traffic below.