It's not just that; mental health is one of the most complex problems available, and it's a big part of disease.
Cancer is hard to fix. HIV is hard to fix. Congenital defects (genetic diseases) are hard to fix. They're easy to identify, easy to understand, and easy to describe; and even knowing everything about them, it's hard to find a way to fix them. When we do, the fix is difficult, complex, error-prone, and severely harmful to the patient. Most diseases are handled by vaccination or by ignoring them until they go away (e.g. flu medicine makes you feel alright until your body gets rid of the flu on its own).
So you think we'll just start picking at these things a bit harder? Okay, sure.
Let's not forget that there's an entire class of diseases you can't even see. When depression or dysthymia kicks in in full force, your trained psychiatrist might not notice. When they do notice something's wrong with you, they're likely to mis-identify what. When they do correctly identify what, they don't have a way to make it stop; they have to go through a huge set of behavioral and pharmaceutical treatments that can affect the disease, diminishing it as much as possible so you can cope. When they manage to find a working treatment, that treatment is unstable, and may fail in the future just because your brain, liver, or kidneys are doing something different, or you're not hydrated as well, or not sleeping as much--as you get older, you sleep earlier and wake earlier, or sleep more, or sleep less, and that can change around your mental health problems and the appropriate treatment.
I'm not saying cancer isn't important; just if you want to take on disease, cancer is not your model for "how hard could it be?"
*A* person with mental health problems doesn't cause much inconvenience for anyone. *A* person with HIV can spread the disease. The existence of all of these diseases, however, places economic strain on our society which does, in fact, make me poorer. Treating all these medical conditions is a waste of time and resources, and could be spent making other crap that our income could buy--that *I* could buy, since the cost of everyone's medical benefit would be lower (cheaper insurance) and thus the price of everything relative to everyone's income would follow, thus I'm able to buy more things. The sum total of all disease does, in fact, affect us all in profound ways, and reducing the impact of those diseases (treating them more effectively, eliminating them entirely, etc.) would make us all much richer.