Because editing a question can change it's meaning, and back (years ago) when it was allowed, a user could change their question to mean the opposite of what was asked, so at that point all the answers they'd received appeared to mean the opposite of what the respondent meant. Additionally, a question could be edited to completely change it so that none of the answers were relevant at all (and thus all could be reported as non-answers). Some users would even post blatant violations, and then edit their question to make it acceptable, after receiving various answers (so it was a way to get around getting a post removed, sometimes).
To stop that abuse, Yahoo changed the system so that Updates can be added to questions if more info needs to be included or a mistake corrected. That way the original question remains. An answer is either pertinent to the question asked or not, so editing them isn't an issue.