The old software was end of life years ago. They are not actively updating it to secure against potential threats. There has already been several hacking attempts on the site and this new software should be more secure. Additionally, there are alot of add-ons that are actively being built for this software. For the old software, there hasn't been any new add-ons for years. One of the examples of a new add-on is the pick'em.
We were originally on UBB. Then around 2004, switched over to vBulletin. The end of life for vBulletin 3.8 series was around 2011. This Xenforo software was made by many of the same people that were building vBulletin and then moved on to create their own company. It's more advanced software and more secure. I think in 6 months, people will look back at the vBulletin and be glad that we made the change.