No DSS means the landlord has had DSS clients who have their housing benefit paid directly to themselves and not the landlord, and instead of handing it over to the landlord as rent, keep it as long as they can until the landlord has to spend time and money evicting them, by which time the tenants have done a moonlit flit and the landlord is screwed over several months rent that he hasn't a hope of recovering. Even if it gets to court, they will have to repay at a pitiful rate.
Unfortunately, there are often a number of personality attributes some people have that can't get employment, and thus being on DSS don't make an attractive prospect for landlords.