IIRC one of the problems was github notifies you as soon as a comment is posted, potentially wasting your time if you go look before the comments are finalized. For example you may see a comment asking how something works, and waste time replying only to see the comment was since deleted because the reviewer figured it out.

I don't think the issues were really that serious, but the Go team likely saw no reason to change away from the tool they were comfortable with for a slightly worse tool that they would have to adapt to.