Comments have less visibility in moderation. This has made them spam / link farming targets in the past.

A lot of people come to Stack Overflow with the mindset that it is a forum to discuss something and have tangential discussions in the comments.

https://stackoverflow.com/tour

> This site is all about getting answers. It's not a discussion forum. There's no chit-chat.

The "no comments until you get a little bit of rep" is to try to help people realize that difference from the start.