Why will Turbo tax allow me to use my 2019 earned income for the Additional Child Tax Credit (ACTC) but not the Earned Income Tax Credit (EITC)?

No.  It's an all or nothing option.

In TurboTax, when you are in the earned income credit section, you will be asked if you want to use last year's earned income to calculate the credit.  If your earned income was greater in 2019 than in 2020, you may get a higher EITC credit by using 2019 earned income. 

For more information see EIC Lookback