Businesses pulling out of the UK based on the Brexit process is bad for the UK, and I'm sure this was indeed mainly motivated by N26's need for a UK banking licence, which is a costly and difficult process to go through. However, it's worth noting that they are pretty small in the UK (~200k users). They're very successful in Germany where banking is a very slow, difficult, manually run industry (from what I've heard)…
So I can see how their model wouldn't be very good, or very disruptive, in places with more consumer-friendly banking options.