The fact is that insurance isn't appropriate for someone who has an existing condition. You pay for insurance "just in case" something happens. You don't go to the insurance company once your house is on fire and ask them to insure it - you're basically just asking them to pay to replace your house.

Health insurance is the same. You ask them to cover medical costs should they occur. This father is fortunate in that he (like most parents) is insured and so when his daughter got sick her care was paid for, but if he was going to go to the insurance company now and insure her for the first time, he wouldn't be asking for insurance, he'd just be asking them to pay for her care.

If we forced an insurer to take on his daughter's case once her condition was already known, they would have to either charge her father outrageous premiums (enough to cover the cost of her care) or increase everyone's premiums to spread out the cost. It might not be a big deal to pay for significant health care services, like chemo, for one or two people - that's how insurance works, after all - but if you force insurers to take on all pre-existing conditions, they're going to have to significantly increase premiums to stay in business and as a result you're going to make insurance unaffordable for a lot more people.