It's fatal because unfortunately today we can detect cancer usually only when it's already advanced - when we do diagnose cancer in it's early stages, the cure rate is quite impressive (e.g. 95% in colon and breast cancer). Current approaches concentrate, among other things, in early detection of cancer.
It's also important to stress out that although we use the term cancer generally, it's actually denoting more than a hundred different disease, each has it's own nuisances and special features, which makes it difficult to develop a cure for all cancers.
You may read more here:
http://en.wikipedia.org/wiki/Cancer 