Viruses mutate, live virus from an immediate host have a far greater chance of overcoming immune defenses, etc. All a vaccine does is give your body the blueprints for antibodies, it doesn't make you 100% immune.
You make a good point. The only problem is, theoretically, if a population is vaccinated against a virus for many generations, and the virus somehow is re-introduced to that population, any un-vaccinated individuals (or maybe even all the vaccinated ones) could be far more susceptible to the disease. A bit like when Small Pox and Measles were re-introduced to the native American population after many many generations of being isolated from it.