Why would a landlord keep a nuisance tenant?

[Ron Jensen]

Why would an apartment landlord keep a nuisance tenant?

Examples are: tenants who party, sell drugs, make too much noise at night, have police called on them regularly, cause problems and generally depress and bully other "good" tenants

Some reasons I can think are:

the tenant is on disability and the rent is guaranteed

the tenant is somehow related to the landlord

the landlord is negligent

Are there any other other reasons why nuisance tenants are not kicked out?