It still had the best reader experience. I don't know why modern email clients, for example, don't look more like trn.
It was also under active attack from Hipcrime and similar.
And no matter how good the killfiles were there were people who wouldn't use them.
I agree about the clients - I really liked slrn.
Nobody really does. The only way you can keep a channel spam-free is to have humans spend time monitoring it to keep the spam out, which is a huge cost asymmetry in favor of the spammers. Anything algorithmic gets gamed and, eventually, broken entirely.