Here is part of the explanation from “Which”
On the face of it, the law seems clear for those who pay for a service, on a credit card, that costs more than £100 and less than £30,000. When a company is in breach of contract and doesn’t provide a refund, the credit card company is equally liable. You’re able to claim the money back from the card provider, which will then chase the company that should have supplied you with the service. But it’s unclear whether airlines cancelling flights constitutes a breach of contract. Their terms and conditions are carefully worded, and allow them a great deal of leeway in terms of changing schedules and cancelling flights.
Read more: www.which.co.uk/news/2020/04/flight-cancelled-can-i-get-a-refund-from-my-credit-card/ - Which?
If you persist you will get a refund, but it’s far from being quick or straightforward. Accepting a voucher for future use may give you more certainty with less hassle.