My armchair General theory is that conscription works better for defensive armies than offensive. Taking a random 18 year old and forcing them to go fight in a different country only leads to questions of why they are there. Convincing that same 18 year old that they should sign up to go fight in a different country out of misplaced patriotism works a lot better. Ironically, that means giving people the choice tends to support imperialism more than forcing the issue.