I think that there is a strong benefit to keep this public or if a vote to make it 51% for acceptance as a core dev.
As we look toward growing the active contributor community and improve inclusion in the core language team, I believe transparency matters more than making these decisions behind closed doors.
By moving this to a private vote, the ability to provide a reason or rationale behind a delay or rejection becomes difficult to identify. This would make it even harder to articulate what an individual needs to do to be considered for core developer commit privileges.
tl;dr Public discourse on promotions adds transparency to the decision making process.