He probably feels like he can't do that, if he wants to be a legit candidate. That's probably what led to this entire episode. Max Blumenthal talked about it on Hill Rising and said he knows RFK and that it doesn't sound like him talking.

Greenwald did an interview on Breaking Points with Grim and Jahinsky yesterday, mostly about this issue, that's quite interesting which points towards RFK maybe changing his feelings, in real time even.