I would like to know why after purchasing a new battery the van will not start.

Maybe it's the alternator not charging the battery.

Sounds like you are saying the starter is barely turning the engine or not at all. If so, disregard. However, if you mean the starter turns the engine good but the engine is not starting, a deteriorating EGR valve could be the assignable cause. If so, you should also be experiencing rare but periodic unexpected engine cut off at lights or while coasting at slow speeds.