So there are various ECB rates, the one that's key here is the deposit facility rate. It's the rate at which banks can deposit money with the ECB overnight. This means if there's tons of excess deposits from customers, this rate applies. It's currently -0.5% so banks have an incentive to not become the bank of choice for cash hoarders, plus there's obvious incentives for many banks to try and push its users to other products such as brokerage which many banks offer in some form.

This is why you find these negative rates in many more countries in Europe. The Netherlands and Germany, for example. My bank charges negative interest, too, above 100k.