The IRS computer system already had a processing to generate direct deposit transactions to bank accounts of taxpayers with refund, so their software engineers could make a quick modification to generate direct deposit transactions of stimulus payment to these taxpayers within the refund process. Then they need to make more changes to their system to pickup those taxpayers without a refund, generate direct deposit transactions if they have account numbers available, system modifications take time and need to be tested to make sure these new functions process correctly, so they could not give a date of payment until they have system modified and tested.
