..and here's the National Minimum Standards document for training for immunisations
http://www.hpa.org.uk...ImmunisationTraining/
The factors I would list are
cost
takeup of the public - you can make the vaccines available but you can't make people attend to receive them
prevalence of the condition
Truth - look at the hoohaa about the MMR combined vaccine, which stopped many parents giving it to their kids, who are now more at risk of these disease
Incoming disease - people coming into the country or returning from foreign lands where communicable diseases are very prevalent
..and that's just for starters