Most diseases are simply treated (aka managed). A few can be cured (so that you no longer have that condition) like a bacterial infection. It may be the case that cancer is only treatable because potential cures would require changing or suppressing fundamental biochemical processes that evolved into our bodily systems (and can't be simply changed or suppressed w/o a radical redesign of our biochemical system).
I'm totally making this up, but if cancer processes were to be discovered to be mostly a function of a rapid partially undifferentiated cell division pathway that occurs when you are a blastosphere that was partly reused in the process to heal skin break or say white-blood cell production etc, etc, it wouldn't be simple to just disable this in your dna before you were born (as that would kill embryotic development). It also wouldn't be safe to disable it completely later because many other things depended on it. The fact that a certain biochemical process must be present to exist in the first place, may have resulted in our evolutionary path relying on the underlying mechanism for many other processes in a deeply nested and intertwined way that might be near impossible to for us to untangle. This may mean that cancer will never really be cured, only treated/managed.
On the flip side, if it were some sort of mutation, or dna methlyization that no critical biochemical system relies on (because it wasn't part of a deep evolutionary pathway), it might be straightforward to just screen for it, or modify dna replication processes to eliminate it, or develop some inhibiting/methlyization factor to markup the dna to avoid the process altogether. That might be considered a cure for cancer.
Today when we say someone is "cured" of cancer, we are really not being honest. The person survived the treatment and they appear to be cancer free for a period of time. The fact is that since we did not actually cure the cancer, it could go out of remission and require more treatment (sadly for some people I knew this unfortunately is not an unlikely outcome).
Of course having a "cure" might be semantical, as a lifetime of management could render it to less critical status (say like type1 diabetes), but if the underlying triggers are part of a multitude of critical biochemical process (because of evolution) it may prove to be quite hard to even have an effective treatment to manage cancer in difficult cases (and/or the side-effects could be pretty bad).