Why are abiogenesis and evolution considered unrelated?
I wouldn't go so far as to say that they are entirely unrelated -- our understanding of selective processes can certainly inform our study of the origin of life -- but I would say that they are quite different projects. 

One important reason is that evolutionary biologists can use phylogenetic information to reconstruct much of the history of life. For example, genetic sequencing techniques allowed us to definitively work out the evolutionary relationships among humans, chimps, and gorillas. But this approach can only allow us to back as far as the last universal common ancestor (LUCA). LUCA was presumably a DNA-based (uni-)cellular lifeform, given that these characteristcs are shared across the entire tree of life. So we have no phylogenetic information that we can leverage to reconstruct what happened to get us from a soup of organic molecules to LUCA. 

Another reason is that evolutionary biology focuses on what happens to a population in which we have variation, heritability, and selection. Early in the origin of life, a structured system of heredity would not yet have arisen and so we would not see natural selection occurring in the form that we usually study it -- though there could be interesting selective dynamics in such a system.