why would turbotax not use the dividend income on a downloaded 1099-DIV form to use in federal tax return preparations ?

downloaded or imported? A download does nothing except create a copy of the 1099-Div on your computer, Import (see the file tab or when you reach the investment income section in Tirbotax), if the financial institution allows it, should result in the info appearing in Turbotax. If there is foreign income taxes there will be certain information that will have to be entered later