America's "shining" women and why it's so hard to cure cancer?

I've been reading Radium Girls: the dark story of America's shining women. It's a heartbreaking story about watch-dial painters in Orange, NJ and Ottawa, IL during the 1920's. Radium derivatives were believed to have restorative properties, even while other's warned of the dangers of pure radium, but the women weren't cautioned or even told about the dangerous materials they were working with, and how they were interacting with them. What's even more amazing is that at least one of these sites is still being reclaimed today, 91 years later. 

Many women and other workers at these plants came down with very rare and unusual cancers. While the video below isn't about radium, it does get at the heart of another issue: why is is so hard to cure cancer?