We would not know the "why"
Only the IRS controls when and if a stimulus payment is distributed.
Go to this IRS get my payment website - https://www.irs.gov/coronavirus/get-my-payment
If you do not receive any other stimulus payments by January 15, 2021, then the stimulus payment for which you are eligible to receive will be entered on your 2020 federal tax return as a tax credit on Form 1040 Line 30.
The TurboTax program will ask about stimulus payments in the Federal Review section of the program.
The IRS will be mailing out payments UNTIL 1/15 so wait until at least 1/25 to make sure a payment doesn't show up.
If you fail to wait and you do file prematurely and a payment does come in after you have filed .... then your return will be delayed, for possibly for weeks, while the IRS corrects the error... so don't be in a rush this year.
The IRS still has not issued a start date for processing 2020 tax forms (could be sometime in February). Last time there was a late tax law change and form changes thousands of earlier filer got caught in a glitch and their returns were either delayed for months or lost altogether and they had to mail them in so the early bird doesn't always get the worm.