Hi there! Yes, I do believe that it was because of his affiliation with the Death Eaters. Slughorn probably accepted him in the slug club but Snape most likely sulked in a corner eating his desert, because Horace wouldn’t talk to him, afraid of what might come out of his mouth. You know. He wouldn’t publicly be proud of a student from his stupid club who was Death Eater. And he wouldn’t brag about Severus because of that. Severus was a difficult person to deal with and (in my opinion) Slughorn liked when students licked his boots or when they were easy going, nice and friendly - like Lily. Severus was the opposite of that.
Lately, only those whom I could not save