[Falkon1313]

It's to reduce security.

Of course a long, random, unique password from a password manager is best for security, everyone knows that.

So forcing people to instead use a short, easy-to-type, memorable password clearly couldn't possibly be anything else but an attempt to undermine the user's security and put their account at increased risk. That bank does not have your best interests in mind. With that in mind, it doesn't matter why they don't.

So switch to another bank (or better yet, a credit union) that does.