I think, like so many other realities of American politics, it needs to be understood as a sad reflection of America itself.
RFK Jr. caters to a specific branch of the mentally ill, but in a country with severe social problems and unavailable mental health care services this is not such a tiny niche. COVID sent a lot of people over the edge, and an anti-vaccine candidate in it's wake is bound to do well.
Combine this with the fact that conspiratorially minded Americans are obsessed with the Kennedys, and you have yourself a candidate who appeals strongly to a non-negligible part of the population.