You dont think the reason people are talking about RJK jr is that we have a D incumbent in the Oval office and a D challenger is polling at 20%? Coupled with the fact that the current President is really really fucking old. As far as RFK jr's popularity. I'd guess it's due to the fact that,
1, He's a Kennedy. That's worth a significant amount of popularity in and of itself.
2. He's a stanch supporter of a lot of D positions. Environment, Middle class erosion, rich not paying enough taxes, social and environmental justice, etc etc.
3. Joe Biden isn't all that popular. I find it hard to believe he wins in 2020 without Trump running against him. The biggest asset Biden has in a reelection run is Trump being the candidate for the R's. Or them being able to cast DeSantis as Trump 2.0. That more than anything will drive people to vote for Biden.
4. RFK jr is an alternative to that. He's something to vote for as opposed to vote against.
Biden is old and getting older. He's clearly lost a bunch of steps. It's actually sad what's being done to him. Not nearly as sad as what's being done to Feinstein but sad none the less. Hopefully we wont see Biden or Trump as the nominee in 2024