Because every Tumblr post is a comment thread. That's how the site works. That's like complaining about screenshots of Reddit comments or comments on a Twitter post having other people's reactions in them.
When a post shows up on your dashboard, it's usually at least 3 steps removed from the original post, and there's 50 different versions with their own conversations happening. By the time this reached Hellsite Hall of Fame, somebody had already commented on it, and somebody else had tagged the hall of fame so that they'd see the post.
The weird part is that none of the users' names are displayed on the post.
People on Tumblr are of course free to comment on anything they like, but the point of taking this screenshot and posting it elsewhere is to show off the original post. I don't see the point in including some random comment if it's not particularly witty itself. This one basically just says "I like this", I don't see what it adds to the screenshot.