I think some people have latched on to things like his Ukraine war stance. Which, while better than most democrats ( extremely low bar), still isn't good. If I recall correctly, he wants to involve UN peacekeeping troops, which doesn't help all that much. Having a position in opposition to the rest of the Democrats doesn't make him a leftist crusader.

He's still not really much different from the rest of the party though when you actually get down to it. I doubt he would even have as much traction as he does if not for his name. HIs zionist position is a non-starter for me just as an example. That's definitely not the only position I disagree with him on.

The more of a stage he gets, the more it seems to push people away from him. I doubt I will be able to vote by the next US election, but I definitely wouldn't be voting for RFK even if so.