Because he was the
happy face of the whole desaster. Claiming publicly how all was fine with very few account holders still having slight problems and repeating this lie ad-nauseum while 100'000s of customers couldn't access their accounts, couldn't make payment and didn't receive any help, whatsoever, from Mr. Pester.
Except lies and bullshit on a massive scale for weeks.
MEPs were also not impressed by his performance.[1]
[1] https://www.parliament.uk/business/committees/committees-a-z...