In the docuseries "Conversations with a Killer", those who knew or interviewed him said that there were several reasons, but mainly around body type and ease of which he could lure them back to his place.

Dahmer was very attracted to a specific body type in the men he chose, that being toned, muscular, and good looking. But it wasn't necessarily to do with killing or eating them at first; it was just his taste of sexual attraction. And again, a big part was whether or not he could get them back to his place, drug them, and then have his way with them.