Probably people who have joined the forum just to access the Ads.
The ones who (sometimes grumpily, often reluctantly) make a few posts, then don't participate in the forum.
Because they don't get involved in discussions they are less likely to feel part of the community.
They may also be people who look at hundreds of ads here and on eBay, Gumtree etc. looking for bargains they can make a profit on by reselling, who don't 'waste time' following up on things they don't want.
Neili