"Why isn't it a Federal tax deduction?"
That is the manner in which TurboTax is programmed. Since miscellaneous itemized deductions are no longer deductible on Schedule A, the program presumes that all excess deductions on termination constitute miscellaneous itemized deductions subject to the 2% floor and, hence, are not deductible.
Guidance has been requested on this issue but, it appears, very little has been forthcoming.
See https://www.aicpa.org/content/dam/aicpa/advocacy/tax/downloadabledocuments/20181031-comment-letter-o...
See also https://www.irs.gov/pub/irs-drop/n-18-61.pdf
If some or all of the expenses (from the total on Line 11A) are not subject to the 2% AGI floor, and you know the amount, then you can enter that figure directly on Line 16 of Schedule A, but you will have to do an override and then print and mail your return.
