Propably because there are a lot of maps submitted here and each time a map is submitted here another map goes further down the list, and basicly when the map is off the first page, nobody will find it anymore. Thus comment on it. And maybe the other reason is the long loading times of wc3search so after people have downloaded the map they dont want to bother come back on the site and wait for the loading times just to comment a map, especially if they dont have much to say.
On worldofwar the list is same all the time, it doesnt change because they have periodic updates, so maps stay on "the first page" for very long time.