I used to work in Travel Insurance, specifically dealing with Medical Expenses claims for a lot of the time.
People who have a history of cancer are, sadly, inherently higher risk for multiple medical conditions in the future. Insurance companies have been caught by this and it's crap for both the ill person and anybody who has to deal with the claim, the angry relatives, the bad press, etc, etc.
The trouble the Insurers face is that, in the past where things could be excluded, is that if something happened, it wasn't necessarily completely separate - not just a secondary or metastases actually happening - I'm thinking of fractures due to a history of long term steroid use, that a more extensive screening would automatically take place in case it was due to bony metastases, that a lump discovered during the examination would automatically be investigated as to ignore it would count as medical negligence, that the treatment has to be different because of previous surgeries or medications, that the susceptibility to infection is different, a cough would be checked in case it was heart or lung related, that some chemo can result in different cancers such as myeloma or tumours or heart/lung/liver damage issues, etc, etc.
And some people (not many, but some) would go on holiday with the intention of getting 'the best' treatment because they suspected there was a recurrence and they wanted it to be covered under holiday insurance (as they wouldn't be able to afford to fly to the US and get the treatment any other way) and would present to the ED with a fall/headache/bump on the head knowing that extensive investigations would result, denying any previous history and it would come out after the Insurance Company had agreed to cover bills.
The specialist companies are really the only option for people who are high risk of making a claim or the potential claim being significant.