I don't think reddit is what actually killed forums. It was a factor, but what was the final nail in most coffins was the software they ran, which often was old LAMP code that needed constant patching and upkeep. Most of these message boards were run more or less non-commercially, as someone's pet project.

They were also extremely juicy targets, not only from a "steal a bunch of emails and password-hashes" angle, but to spam with links for SEO-purposes. I think this constant attrition is what ultimately killed them. It simply became untenable for one person to stem the tide. Reddit lives because it has the economies of scale on its side.