Humanitarian visas can be obtained when a person that needs medical treatment can only get it in the United States
Humanitarian visas are typically only granted in life and death situations.
Because there are thousands of people that ask for this type of visa, it cannot be given to everybody.
It is also important to keep in mind that someone has to be willing to pay for the medical expenses.
The United States won't give a Visa to everybody that is sick, and won't pay for their medical treatment.