Costa said that because the book was an abstract produced in haste in response to Wallace's essay, it was more approachable than the big book on natural selection Darwin had been working on, which would have been encumbered by scholarly footnotes and much more technical detail.
Why did James T. Costa think that On the Origin of Species was more likely to draw interest than a larger book that Darwin had been working on?
because the book was an abstract produced in haste in response to Wallace's essay