It's possible that RFK was pushed into running thinking he would take more Trump votes than Biden votes. When it turned out to not be the case, they decided to have him drop out instead?

Normally, a prominent 3rd party candidate takes more votes from their closest main party candidate, throwing the election to the other candidate. I thought RFK would be stealing more Democrat votes than Republican votes. (Example: Perot cost Bush the election in 1992. Nader cost Gore the election in 2000.)