There are so many reasons people with mental illness do not receive the care they need, and they are all, in one form or another, barriers to mental health treatment.
First, mental illness is stigmatized, certainly in the United States, and possibly in other countries, too. There is a tendency, because we have little in the way of physiological markers for diagnosis, for people to think that mental illness is not "real," and a concomitant tendency to blame the victim, whom people believe can exert some control over whatever the problem is. The consequence of this is that people are often reluctant to admit they might be mentally ill, and this prevents or discourages them from seeking treatment. So, our culture creates a barrier to treatment. As time goes on, I am fairly certain that there will be an understanding that mental illness is a biochemical disease and that we will be able to diagnose it, maybe with brain scans, or even with something as simple as a blood test. Given the lack of compassion I have seen over the years for people who are mentally ill, I cannot help but hope that something will allow people to see that this is a real problem.
A second reason that people have been reluctant to seek mental health treatment or to follow up after they do so is that insurance companies have not always provided good coverage for mental illness. The new healthcare statute (Obamacare) addresses this problem, and anyone who cares about the treatment of mentally ill people should hope it is not repealed. The insurance companies' failures include inadequate coverage for hospitalization, which can be crucial and lifesaving for people who are mentally ill, for example, with severe and suicidal depression. But also, insurance companies have a tendency to like to cover medication and not therapy. Pills are cheaper than therapy. However, there is good research out there to show that mental illness is always best treated with medication and therapy, and that people who only take pills often relapse or are never completely "whole."
A third reason people do not seek treatment is that there might be few options for treatment, long waiting lists for appointments, or a complete lack of knowledge that any help at all is available. When a person is mentally ill, he or she is likely to be paralyzed with psychic pain. It takes a great deal of energy, intelligence, and patience to navigate the mental health system, supposing one lives in an area where there are any resources available.
Finally, there are still millions of people who do not have any health insurance at all. Many of these people are not eligible for Medicare, or they are too ill to jump through all the hoops that Medicare requires. For those people, sometimes it is a choice between eating and seeking treatment, just as it is a choice for people with physical illnesses. So, this, too, in an often insurmountable barrier for the mentally ill.
It would be wonderful if we could de-stigmatize mental illness, make sure all of us had reasonable insurance coverage for it, and reform the system, so that people who are mentally ill can get help with greater ease. There are organizations that work toward these goals, but, sadly, they have a long way to go.