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.
And yet you have 29622 karma points here. To accumulate those points, how many hours did you spend reading and writing on what you'd presumably call a ghetto/silo?
Mailing lists are fine. I run a few, out of a box under my desk here. They do work for long discussions.
Usenet and e-mail are closely related; there is a reason for Usenet to exist.
There exist gateways between usenet and mailing lists, too.
If you join a new mailing list, your only option for "back issue" content is to go through an archive. Either a web archive, or else mbox files.
Usenet has a pull model. You connect your client to a server that stores articles going back whatever: months, years. You're not subscribed to receive anything spontaneously; if you want to quit Usenet, just exit the program, and that's it.
Numerous newsgroups are available in one place; you can search through them in your client and take a look. Mailing list archives tend to be scattered all over the place. GNU mailing lists here, BSD mailing lists there ...
On Usenet, you never have to use an identity with a real, working e-mail. Not so with mailing lists; they don't work without a reachable e-mail address.
Usenet has spam and so do mailing lists. But mailing lists have actual e-mail spam. Not only are lists sometimes the targets of spam, but spammers harvest addresses.
So, when I subscribe to mailing lists, I use throw-away addresses, unique for each list. This is a dance I do not have to do with Usenet.
When you use different addresses for subscribing to different mailing lists, you have to be careful to also use the matching addresses as your posting identity! (If you use your principal e-mail address by accident, it could get harvested. Or, something else: the posting might not go through to the list at all, due to that address not being a member.)
When a throw-away mailing list identity receives spam and you replace it with a new one, your old identity still exists in ongoing discussions and will be CC:'d in new postings.
You can use a working e-mail address in your Usenet From: header, if you choose to do so, so that you can be privately contacted. Getting a private e-mail reply from a Usenet posting is rare. If that address starts getting spam, it's pretty easy to change, just in your NNTP client setup. The address is not the basis of your NNTP identity, and so changing it doesn't break threads.
It doesn't involve your mailbox in any way. You don't have to register. "Subscribing" to a newsgroup is purely a local configuration, and you don't have to do that to post to it.
When you sum it all up, Usenet is easier to use and more casual than mailing lists. That gives it a different flavor.
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.