Ms Pierce (50) said giving greater executive power to members 'gave me the opportunity to go'.
'People like me who have had a high profile would have less profile. I'm not sure I'm up to changing in that way,' she said.
A London colleague described Ms Pierce as having been 'on the 'at risk' register for some time', due to some Tower Hamlets members being uncomfortable with her ebullient management style.
Ms Pierce said she had an excellent working relationship with Mr Keith and claimed problems with members had played no part in her decision to leave. 'It would be a strange world in local government if you never had any difficulty working with councillors,' she said.
'It's not about that, it feels much more positive . . . If anything, I'm critical of myself for not being prepared to change.'
Ms Pierce will take early retirement at the end of September. She said she had received no offers of employment, but wanted to continue working - although not in local government.
'I can't see myself sitting at home knitting. If you're going to be a chief executive in local government there's nowhere more fantastic to work - I'm really sad to be leaving.'
Developing private sector relationships had been a major highlight of her time at Tower Hamlets, and a role that fused public and private sector work would be ideal, she said.
The council said Ms Pierce had been 'instrumental in bringing resources into the borough and enhancing its profile and reputation'.
It said it anticipated a 'changing role for the chief executive'.
Ms Pierce is credited in particular with bringing£600 million of private sector investment into the borough and forging a strong relationship with the Canary Wharf development's management.