The reviewer mentions Gold in order to:
A emphasise the wide range of topics that Hurst covers in each book.
B explain what he sees as a weakness in Hurst's approach.
C) support his opinion that Birdlife deserves to be very popular.
D) express his disappointment with Birdlife in comparison with Gold.
B is the right answer.
You asked why C is not the right answer. The reviewer clearly does not think that Birdlife deserves
to be very popular. I suggested you read the piece because I hoped you would see these things, but you didn't:
The reviewer says that Birdlife is "in some respects ... welcome". That is qualified (less than total) praise. That should alert you. Then he mentions Glenda Hurst's previous book, Gold. He says that book lacked continuity, there was no sense of the the author presenting a case, overall it was lightweight, just a series of magazine articles. THESE ARE CRITICISMS. "Nevertheless", the reviewer says, the book sold in large numbers. In spite of Gold being a bad book, plenty of people bought it. He says that Hurst chose to repeat the formula (that is, Birdlife is as bad as its predecessor). He finishes by saying that Birdlife should (i.e. probably would) sell in large numbers for the same reasons that Gold did (because there are plenty of uncritical readers ready to buy lightweight books).