Why not Forums?
VBulletin/phpBB style discussion boards are great, and there's usually one for basically-whatever you're into - cars, computers, pets, food, crafts, wedding planning. It's not uncommon for a given user to be a part of at least one such community. The problem is that it's a bit more difficult to 'channel surf' that way. If a user participates in a discussion thread in Alpha Forum, they'd have to log in separately to Bravo Forum to participate there. This limits the scope of topics that can be viewed at a given clip. Similarly, different forums have different rules or customs. Some are particularly strict about citing sources for claims, others are super strict in the profanity, others use a 'reputation' point system while others simply use 'thanks' or nothing at all; Reddit is at least a smidge more consistent site-wide with its rules.
Why not Usenet?
I'm still a fan, personally. comp.misc has some interesting discussions, and misc.legal.moderated is frequently some fascinating reading. There are a few groups related to video games and Doctor Who that have interesting discussions, the latter particularly after new episodes air. The fact that a user identify is basically universal is helpful to keep the playing field level. Usenet has its own list of problems though. First and foremost, usenet is something that itself needs to be sought after to a certain extent at this point; few ISPs offer access to it, and neither Windows nor OSX ship with a client. This is further complicated by the fact that a "binary client" and a "text client" are frequently different pieces of software, usually cost money on top of existing internet subscriptions, few real-life friends tend to be able to share the experience, and the search results for the term in Google are usually affiliated with warez, so getting people into it is its own challenge to begin with. Moreover, usenet is much more susceptible to spam, it's not possible to share in-line images or youtube clips (like it or not, responding with memes and animated GIFs is a common practice these days), and it's not possible to 'upvote' posts as to indicate the quality of a particular contribution. Put it all together, and Usenet is a wonderful niche that Reddit simply does better for the majority of internet users.