Answering another commenter's question, I would say the "secret weapons" are:
1. Hiring: Haskell is a very in-demand language by very good engineers. For a startup, it's absolutely amazing for recruiting and I can't overstate how important recruiting good people is for a startup.
2. Correctness: Haskell helps you ensure your code doesn't have bugs. For a bank this is fairly important. Some aspects of Haskell that are great for this: algebraic data types ("discriminated unions" in some languages) model business logic quite well, purity makes a function much more of a known quantity, libraries like Persistent ensure type-safe SQL queries, and in general the community cares about correctness.
I haven't used the other languages you mentioned, so I can't really speak to a comparison with them. Re: laziness, it's almost never something I think about in practice.