Because they have no answer to state violence, what's the demsoc response to the CIA for instance? Whatever it's bound to be, it'll never be enough and they'll get their people hurt or worse
But they're still useful as a potential demonstration effect and popularizer of socialist thought, in a situation where legalism is state mandated through overwhelming violence like in the imperial core, demsocs can function as an effective stopgap measure, until better conditions arise for more serious tendencies to gain traction