I think the problem is likely to be, not that you have cancer, but that you are under investigation for a medical condition of any type. Until an insurer can have some certianty, they will not take the risk. We wanted insurance for a short trip to Germany, but my wife was under investigation for a stomach problem, and no insurance company would quote. Not surprising, really - they are being asked to quote for a situation whch could potentially leave them open to very high medical costs, since your diagnosis and prognosis are both uncertain. Might seem a bit harsh, but insurance is all about weighing risks against costs.
Hermit