Everyone dies eventually. The deaths that would have been from whatever is no longer killing us are being taken over by something else that still does.

Or, another way to look at it: we are living long enough now that we have a much better chance of contracting cancer during our lives.

There may be myriad other causes (on which I'm largely ignorant), but one reason cancer kills more people than it used to is that more people are living long enough to be killed by cancer instead of something else.

It frequently takes a fair amount of time for the various micro-level problems in cells to develop into cancer, and then it takes some additional time for that cancer to get bad enough to kill you - which is why young people rarely die of it, and one reason why "death by cancer" will be rare when everyone dies of something else while they're still too young to have had much cancer risk.

There are a few possible explanations for this, although three obvious ones jump to mind:

1) There is more cancer today or the cancers are more lethal.

2) We are better at diagnosing cancer today.

3) We've eliminated (or dramatically reduced) a lot of the things that aren't cancer that can kill you.

My money is that it's mostly 2+3, and that those have brought about a bit of 1.

The longer you live, the longer you have to get cancer.

People live long enough to have cancer.

Because God ran out of other ways to kill people... And everybody still has to die of something, sometime.