There are two reasons for that. One is simple logistics. When you need to train a large body of folks, typically on multiple shifts, it's pretty expensive to try to do that with outside consultants. The second reason, probably more importantly, is it's really part of the ownership process. Employees will be more likely to support a program when its benefits are explained by one of their respected peers.
Timing is everything.
Your ISO 9000 training should occur close to the time when the new procedures are scheduled to be rolled out. If you train employees too far in advance, they may lose interest and forget what they learned. If you are not implementing all of your procedures at the same time, train just those employees that will be affected by the changes at each stage of the implementation. Keep it short. Kausek recommended spending no more than two hours on the ISO 9000 introduction.