The old forums used processes and bandwidth that kept making the server that had the forums (and the website, if I remember correctly) keep crashing. It was a php based format, which isn't bad when you don't have 200+ people visiting you every day/week.

This is a different format (ColdFusion, I believe) which is a lot easier on the bandwidth and processes and all that hullaballo.

If you want to learn more, run searches for all the threads that complained about the new forums.