Well, they didn't "have" to, it's just the Spanish parent bank didn't want to pay a competitor £100m a year to use their software system and mandated TSB move to their amazing, all new, all singing, all dancing, software platform.
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]
Obviously if they were force to move they'd go to a new fangled IT system, not adopt some old tried and true system (those are too tied to the banks that developed them).
Quite why that means the UK head needs to go ....