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The main reason is that they are not considered as players. Yell is a pay to play outfit that IMHO provide next to no traffic for the money you spend.
Worth it if they charged £5 per annum, otherwise forget it.
However, they have something like 2 million business listings (less than Thomson Local) with which sites/businesses have to compete. That's an awful lot of competition for placement.
They charge £119 to upgrade a listing for 12 months, but the standard listings remain free I think. Someone may be able to correct me on that.