1. The clients were kind of a pain for anyone but comp.sci nerds
2. It was butt ugly. Web forums killed it, because you could make them pretty
3. Not only spam, but the troll bots just destroyed everything. I can't remember why exactly this started happening, but there were a dedicated pool of assholes spamming Usenet groups they didn't like with garbage or vile crap posing as regular contributors of the groups. When you searched for posts by someone, the first 100 pages were from these troll bots