Darwin gets the credit because he put all the pieces of the puzzle together. Wallace did the same, though later. Even though Darwin hadn't formally published, by the time Wallace's ideas came to the attention of the British scientific community, Darwin had already established priority for his ideas through a voluminous correspondence.
Darwin also gets the credit because he presented, explained and interpreted a large body of evidence supportive of evolution. While Darwin included and built upon the idea of the mutability of species, he never claimed to have originated the idea. As in all science, he built upon the work of other scientists.
As the years went by after the publication of
Origins, one of the criticisms Darwin received was that he credited too few people, and the list of acknowledgements grew longer with each edition.
It isn't necessary or even valid to malign the popular image of Darwin in order to discredit the idea of evolution because, as we all know, Darwin recanted his theory on his deathbed. Since this is a know fact, and since it has been scientifially established that deathbed recantations take priority over issues of evidence and interpretation, obviously the theory is false.
--Percy