Ignoring the Security concerns, keeping in mind that the password text is not visible (just asterisks/dots), a couple major UX reasons I can think of are:
Depending from where you are copying the password and where you are pasting it, you might end up with messed up clipboard entries (changing text from utf-8, html, richtext, docx, etc or something else).
Another common mistake will be copying empty spaces.
- Frustrated user who cannot understand why his seemingly correct password is not working.
- People thinking they were 'hacked' and their passwords changed.
- In situations where only limited attempts are allowed, this can result in the account being locked.
There are ways of getting around this problem, like, letting the user toggle between a visible and obfuscated password text. But even that is not completely foolproof (Many input fields have padding which shows up as whitespace before the first character, an empty space can still be added there via copying. Similarly, spaces can also be added at the end if not careful.)