The shifting of the distribution of health risks that is the point of the population model of prevention is very difficult to achieve because it

It's difficult to achieve because of the target is for the whole population not only the participants who would like to be involved. Aiming for the population could be difficult because there are different factors that would become barriers and influences which can affect the decision of the people in the population. Tracking of diseases can also be difficult because people can transfer and live in other areas without notification. It requires a health database that can sync clinical status and histories. 
Source: https://www.ncbi.nlm.nih.gov/books/NBK43749/