Many times, people try answering questions in comments when they're unsure of an answer rather than trying to say "this is the answer" and create an answer of their own that is sufficiently in depth to be helpful to others.
Other times, new users try to ask questions on the comments of "I'm having this problem too, do you have a solution yet?" or, while they do try to ask for clarification on the question - do so on questions that are stale.
Lastly, comments were often (in the days of 0 rep comment permissions) targets of spam that could go undiscovered.
The way that SO decided to solve these problems was to add a bit of friction to leaving a comment. This meant that users who were trying to contribute answers were directed to answers; users who were trying to ask questions on stale content would... get frustrated and go to a question where there is sufficient information; and automated spam would be that much harder (requiring an investment of time from the spammer).
It's not an ideal solution, but from the standpoint of a site with volunteers curating and moderating the content, creates fewer problems for the people who invest the time than other solutions do. Burning out those people who invest the time is one of the things that has concerned people in the company concerned about the existing community - losing them would have SO devolve into something that more closely resembled the software development section of Yahoo answers... or https://www.answers.com/t/computer-programming
