For the simple reason that I've never used this forum as a business opportunity, with the debatable exception of my infamous Merit Cycling Club, though that deal is still ongoing so there's technically still a counterparty risk, even though I trust everyone involved.
Aww, that's kind of you, Foxpup. And the reason you gave was what I was going to suggest if you hadn't posted here--I've never seen you do any deals on the forum, so there would be no real need for you to hand out any positives. And man, there are just so many reasons why members should get negatives.
OP, you'll find that DT members tend to have a very high negative/positive ratio for trust left for other members, and that's because a lot of DT members have become trusted by busting scams and catching cheaters. They also have to be very conservative about leaving positives, since that could potentially turn a questionable member's trust to green--and that could be exploited in order to scam. It's been known to happen before.