The response "they're only doing this because they can't make enough money in the UK" seems kind of vacuous. Of course that's why they're doing it. The cost of requiring a UK licence outweighs the benefit of being there.
There seems to be some sort of desire to say "Brexit didn't cause this". It's pretty clear it did because they could previously operate without a UK licence and now they can't and it doesn't cost low enough to be worth it for them. I don't think it matters in the big scale since there's Monzo and Starling, but having the EU fracture will be annoying since markets will fracture.
As for the criticisms of starting in the UK post the Brexit vote, I don't understand if you guys have run businesses before, but you have to take risks. Things can change, and for the right growth opportunity you go for it.
As an example, post-GDPR I set up the European arm of our data business, and even though Brexit was on the horizon, we set up in London. Why? Because the opportunity was big enough and we could work our way to a solution in time that would meet what we needed and because (being entirely Anglophone) establishing a bridgehead in the UK made sense. It's a successful business now and we have teams in mainland Europe now. It's like running towards one of those closing doors so that you can slide under it like a hero. You don't want to be trapped under the door, but sometimes what's on the other side of the door is worth the risk.