Because it sucked, and people fondly remembering it have blotted its manifold failures out of their memory. Imagine HN or Reddit, but virtually every post in every thread quotes huge chunks of preceding posts, often many layers deep. Randomly remove comments from threads; remove different comments from different servers at different times, to make sure nobody sees the exact same content. Eliminate search. Eliminate all but the most blunt-force moderation. Eliminate user profiles, or following particular users. Divide the universe into a global taxonomy of discussion topics, then allow people to cross-post from one to the other, starting firestorms of confused argument and complaint. Wait 2-12 hours for your discussion partners to see your as they move peristaltically through the store-and-forward NNTP system. Expire all history after a week or so. Every once in awhile, just for fun, drop a 50-comment-long chain of uuencoded binary chunks into the middle of your debate about Lisp.

Usenet was one of the first systems of its kind, and it's no wonder people look back on it wistfully. It was wild, being able to talk (or argue, or flame, or troll) people all around the world at every hour of the day. Usenet was amazing. So was the Sony Vaio. You wouldn't want to use it today, though.