Aaron Margosis who writes for the Microsoft Security Guidance blog on TechNet addressed this specifically a couple of times regarding the change to "Not Configured" for Secure Attention Sequence aka CtrlAltDel. It basically boils down to the fact that users can't tell if they're at the secure desktop, it's easier to steal the same user credentials inside the desktop, and it's hard to implement on keyboard-less devices.
Changes from the Windows 8.1 baseline to the Windows 10 (TH1/1507) baseline (20151118)Aaron Margosis November 18, 2015Enforcement of Ctrl+Alt+Del at logon to protect credentials from theft."This is not particularly strong protection. First, it depends on a user that’s looking at a spoofed logon screen remembering that he or she hadn’t pressed Ctrl+Alt+Del before typing a password. Second, so many apps prompt the user for the same credentials on the user’s desktop that the credentials can easily be stolen there. Third, if the adversary has gained administrative control of the computer, the “secure desktop” is no longer a protected space. Finally, with devices offering more keyboard-free logon experiences such as facial recognition, Ctrl+Alt+Del becomes an annoying interference."
from "Security baseline for Windows 10 – DRAFT" by Aaron Margosis, October 8, 2015
"[Aaron Margosis] Great question. One is the increase in systems where a Secure Attention Sequence (SAS) isn't feasible. Second is that the SAS has probably been a very low-value protection, overall. See the discussion about a setting where we made it possible to go overboard on the SAS and how that didn't really work out well:"
