RFK is doing it because he's thought vaccines were bad for literally his entire adult life, here's an article from 2011 where Salon had to retract a thing they let him publish in 2005 on the subject:
You're not required to bring everything you've believed your entire life to your political campaign though. In fact, it's better not to. You're there to do a job, not get a prize for believing things.
I have no side in this debate because I don't support any of the potential candidates, but since US politicians are not effectively disciplined by their base but rather by their donors, it's quite reasonable for the base to seek evidence of past commitment to issues they prioritize. It's a very poor heuristic, but it's all they've got. As you said, the politicians are doing a job, but they don't take their orders from the voters. The system is utterly ruined.