It would be interesting to hear Google's actual reasoning but I don't expect that we will. I will speculate that it is exactly the clipboard permissions as there have been apocryphal reports of Android apps and web extensions that use this to steal passwords that password managers put there for users to "paste" into their pages.

If that is the case, then a much better solution would be for Chrome to implement a secure channel for password managers to use for just that purpose and make access really really explicit. But again, without them saying anything we won't know.

My advice is to watch for a CVE regarding sniffing sensitive data off the clipboard to surface in the next 30 - 90 days.