I don't think anyone has mentioned it, but could their motive for this launch is to get in before Monzo launches in the states (source: https://www.forbes.com/sites/oliversmith/2018/10/31/with-a-f...)

Monzo has blown up in the UK, and I know their team is growing at a crazy rate. While they aren't profitable yet, they plan to sell financial services through their marketplace (insurance, mortgages etc.) which seems to be a major source of potential income for them (source: https://monzo.com/blog/2017/11/16/monzo-marketplace/)