Doctors are, in general, risk averse. The higher the perceived risk and the younger, healthier the population a given doctor deals with the more risk averse they will be. Pediatricians and OBs are, again in general, especially risk averse and take a long time to get on board with new treatments. It takes a binding UN resolution to get an OB to agree to give a pregnant woman almost anything for example. Things we've known for decades are safe for pregnant women still make them cringe.

Now all that being said, this is a concern not without merit. Not so much because of the risk of the vaccine, but because the potential benefit for 5 year olds is so incredibly low that it almost isn't even worth the time and money to go have it done. No matter how low the risk of a therapy, there needs to be enough of a benefit to justify it.