Probably because few people check this sub forum. I check here several times a day and there are never any more than four or five members viewing this sub forum at the same time I am. Usually more like two and one of them is me.
I sometimes see knives listed in The Exchange that are also in this Wanted section. The seller obviously didn't check here first, which is fine since nobody is obligated to do so. When I notice it I try to contact the person looking for that knife and link them to the sale thread.
Personally, I like checking here because if/when I find someone looking for a knife that I would part with it saves me the trouble of monitoring a sale thread. It's a quick sale for me and the buyer gets what he/she wants too.
I think another reason might be because when a seller does contact someone who posted here they don't always reply in their thread. I'm guilty of that myself. I've sold at least a few knives that were posted in this Wanted section and I rarely, if ever, reply to the original thread. I've contacted the potential buyer by email or PM so there is really no benefit for me to post here. It might be a good idea for the buyer to post here that the knife has been found but that is their choice.
I like this sub forum but yeah, it just doesn't seem very popular.