Because a hardholder has more to lose. If Keeler loses her gang, she can roll with it - so what, they were pissers anyway and who needs 'em, she's still a gunlugger. If they get shot up, that sucks, but it's also what they signed on for when they decide to hang with her. If a hardholder loses his hold on the people in the holding, it blows who he is wide apart, and could cost him the holding. A hardholder can't rule by fear alone, or the very stability he's worked for is at risk.
A gunlugger might care for his gang and build on mutual loyalty, and a hardholder might control her holdfolk through webs of fear and intimidation, but there's definitely a different vibe .