While part of me would like to think it is just because the big ones being researched right now are just not easy to cure, most of me fears the greed factor may play a role.
I am certain there are lots of scientists and researchers out there who would like nothing more than to cure the diseases they are studying. But the labs and companies are run by administrators and executives who answer to shareholders who are more interested in ongoing profits.
So if they invent 'maintenance drugs' instead of cures, they can look forward to making billions for years to come. I hope I'm wrong, but I fear it is all about greed.