Why does 2018 Schedule D information prepared on Turbotax not automatically populate 2109 return?

It depends.  If you are referring to a Capital Loss Carryover it should bring that forward.  If you did not use TurboTax in a prior year and/or your  carryover was not brought forward, please first double-check your last year's return to be sure it wasn't used to offset capital gains, or otherwise used up. 

If you need to type it in yourself,  you can type capital loss carryover in the search box at the top right of your screen and choose the Jump to link.  See the screenshot below.

Generally, for most taxpayers, other information related to Schedule D for Gains and Losses is unique each year, so Turbotax does not carryover that information from year to year.