Yesterday I read a post where the author admitted that if people believe it's an advert then it is because they want great people to come and work in the company.
Non tech centric people go with options which are cheap and just work -> MS products. You buy windows, you buy 365 you buy Azure , makes things cheaper. IT product centric companies focus on quality which you get on AWS.
No doubt most banks would just go with MS products but fintech banks are with AWS - n26, revolut, transferwise (not a bank though).
However it's the best not to rely on any single provider and keep your eggs in different basket. But AWS is the first choice when you think about quality. MS the first when you think about cost cutting.