Essentially it was because of the impact of Origin ofSpecies.
With their joint paper, Darwin and Wallace can be thought of a co-proposers of evolution by natural selection. Unfortunately for Wallace’s fame stakes, this joint paper did not arouse much interest at the time. Origin, a year later, with Darwin’s name at the forefront and Wallace being deferential to his colleague, captured both scientist and public imagination. From this, Darwin was the one being ridiculed in cartoons as a half-ape, and Darwin was the name people associated with evolution. It’s also interesting to note that natural selection (but not evolution) went through somewhat of a out-of-fashion period in the early 1900s, which affected Wallace’s fame while not as severely affecting Darwin’s, whose fame stemmed from bringing evolution as a whole to the attention of the world. Later, the Modern Synthesis, a sort of union (or reunion) of evolution, natural selection and genetics in the 1930s, seems to have remembered the contributions of Darwin whilst largely forgetting Wallace’s. As we inherit this synthesis of evolutionary thought and its associated history, we cast Wallace’s role by the wayside – a fitting and ironic example of cultural evolution and survival of the fittest.