80% of the chicken population is estimated to carry Marek's, with the majority being asymptomatic carriers
The main breeds that are well known for being suspectable to Marek's are those with high levels of inbreeding (Silkie, Sebright, Orpington, etc.) Humans of course breed for visibly obvious traits and not the immune system
The fact that it's your pure and line-bred chickens only adds more evidence that the problem is genetic in origin
Natural selection and more outbreeding seem to be effective solutions to the problem