[chomp]

Not parent but I'm guessing the rationale is that a password manager could undermine the concept of 2fa.

Some believe that the "something you know" should be stored inside your head. I personally use a password manager, but can understand the viewpoint.

Password managers lie somewhere between 2 different factors, "have" (the password DB) and "know" (only your master password). For those who use a laptop as their 2nd factor (yubikey plugged into a USB port, a token on the system itself) and get their laptop stolen, a compromise of the password safe could result in both factors being breached.