They're very successful in Germany where banking is a very slow, difficult, manually run industry (from what I've heard), but in the UK we have generally very good banking infrastructure – payments are normally instant and free, and there's a lot of consumer protection with things like the current account switching service guarantee.
Startups such as Monzo and Starling started in this great infrastructure and now offer relatively compelling products, while the incumbents are getting to grips with modern tech and doing better than many people realise.
N26 on the other hand just didn't have a compelling offering. They charged for most/all of their accounts (explicit charges for accounts in the UK are uncommon), they lacked features, and their marketing was unremarkable compared to the competition.