Because no matter how terrible a person they were, they're dead now. It's over, and there's no point in stirring up bad memories. By continuing whatever feud you had with them after it ceases to have any purpose, you're only making yourself look bad.
Though of course it's really more of a guideline than a completely hard rule, and I think it's mainly intended to apply to personal disputes rather than, say, mass murder. No one cares if you call Hitler an asshole, but stop badmouthing your estranged grandfather, ok?