Every frame is still a full HTML document that needs to be generated even if it just contains a single comment. This page has 370+ comments which would mean 370+ additional HTTP requests to load them all as standalone HTML pages. That's a massive increase in server load.

On the client, iframe tags are entirely separate browser windows and can use up considerable resources. It's like opening up hundreds of tabs and is far more intensive than running some javascript code which is quickly parsed, compiled and cached in modern JS engines.