I was running my site on O'Reilly Webboard, adn the load was killing the server. It was single threaded
We were loosing users because of this and so we as a site started looking at alternatives. The open nature of vBUlletin was the biggest plus. After 3 weeks hacking in the Persistent Mark Forum Read hack, to make it feature comparable to Webboard, we went live.
Other products we looked at included Panfora and Web Crossing.