What I don't get is why wouldn't they be able to credit /debit regular bank accounts with money instead of creating a new "digital system"?

I'm fairly noobish at understanding all this stuff, but as I understand it there are severe structural issues with the existing banking system that make a completely new mechanism desirable. For example by depositing cash into traditional accounts, it is also equivalent to providing capital to traditional banks to loan from. It would therefore be difficult to supply cash to end users without also increasing the availability of credit, which may be undesirable for any number of policy reasons.

the most obvious case is that people don't have a bank account or are 'underbanked' in the US that amounts to about a quarter of the population[1]. I think covid has shown what a mess it is to get everyone their relief money. If everyone had a digital account with the FED it'd literally be a few clicks of a button.

[1]https://www.cnbc.com/2019/03/08/25percent-of-us-households-a...

Because central banks have no direct control over regular bank accounts? Regular bank accounts are not held at central banks (yet).

So that they can have restrictions on how the stimulus money could be spent.