I think it is based on the concerns some parents have regarding any autism risk involved. While many say that the vaccine has been proven safe it would be more acurate to state that there is no clear connection between autism and the vaccine and some studies show little correlation. There is no absolute proof that there is no connection.
Maybe it is time to have a Kennedy in charge of people with the right knowlege and credentials look into this. The powers that be till now included Fauci, a quack that made millions selling his own superfluous vaccine and was chairman of the vaccine commision in the USA. His vaccine was not recalled when it wasfound to have live virus contamination.
Vaccines are made with live virus and anything can happen in any batch that might affect those that use that batch only.
It should also be noted that often these vaccines are sold in multi dose bottles and the bottle gets old and possibly contaminated. By comparison, flu vaccines and dental anesthetics are single dose in most cases. This shows that the cost is not prohibitive for single doses but some pediatricians are more concerned with cost than risks.