Because Vito treats the residents with a great deal more respect than Fanucci had, he soon earns the neighborhood's loyalty. Since Fanucci was not well-liked by the police, his murder is not properly investigated and is eventually written off as an assassination committed by a rival gangster.
Vito's kinder attitude towards residents leads to neighborhood loyalty. In contrast, Fanucci's unfavorable status with law enforcement means his murder isn't thoroughly investigated and is attributed to another gangster.