let's start here (medicine clueless here so excuse any massive mistake)
there are MANY types of cancer, I imagine that while the obvious effect is the same, the specific causes of each are different.
Now, I remember a while back that they developed a vaccine for a specific type of cancer (cervix cancer).
So to start with its not as simple as saying 'there is a cure for cancer'
Also applying this same type of 'logic' there could be cures for just about every disease and the 'logic' would still apply.
Finally, I believe that if they had anything that would remotely resemble a 'cure' they would be very actively selling it and not keeping it in the dark after (presumably?) they spent a lot of $ in developing it
Its far easier to say something like, hunger in the World could be erradicated, that is most certainly possible but it doesn't happen for many reasons, in the mean time, on this side of the World we die because of the precise opposite.......eating too much crap
one more thing, a good 'cure' for cancer is not smoking...yet people don't care so why do we need conspiracies here?