Unlike, say, TripAdvisor (where anyone can add a hotel to the review listings), Trustatrader.com requires traders to sign up to the website. The process requires a minimum of 5 references, together with copies of insurance documents and trading terms. More importantly though, traders on the website have to pay a fee of around £500 per year for membership. It seems likely that the traders you were seeking have decided that membership is not a worthwhile investment for them.
Traders using Ratedpeople.com pay a lower membership fee (£15 + VAT per month) but they then have to pay for every lead sent to them. The cost for each lead varies depending upon the size of the job but it averages around £15 (+ VAT) per time. So Ratedpeople.com seeks lots of information from you in order to make about £45 out of forwarding it to three traders.
Chris