Are there people more susceptible to scabies?

Probably . It is probably related more to behavior that increases risk. This is not proven.
Some get bad case. More specifically people with immune system deficiencies can get a much more severe form of the disease with a greater burden of mites and more inflammatory changes in the skin. This form of scabies is called "crusted scabies". For the most part simply acquiring the disorder is just a matter of exposure to someone with the disease.