This has never been touched upon, I keep hearing Scarfo was afraid Salvie would
take over etc but having heard Phillip say "my uncle said if I'm dead u gotta keep this thing going", everyone assumed Scarfo was afraid of Salvie's power. Personally I think he knew Salvie wouldn't make a move on Scarfo himself BUT...

without Scarfo around (dead), he knew Salvie was going to be a threat to Phillip taking over. Not saying Salvie would make a move on Phillip either, just that with his status and following, there would be a strong case for Salvie taking over the family instead of Phil. And why did Scarfo want Phil to continue? Money. Apart from the LCN aspect, running a family guaranteed a steady income for he entire "personal family" therefore IF Phillip does well so do Scarfo's immediate family

Same way the Snake ordered Wild Bill's murder. Nothing to do with the beef they squashed, more so to do with keeping the money flowing Persico side (albeit different circumstances somewhat)