My guess is that there may be backroom deals going on since the beginning. There are plenty of cases where an act's last 12 month track record played no roll in NHK's inviting them.
My favorite overlook by NHK was Momoclo and Momoclo's defiant reaction of creating their own live New Year's Eve show and morphing it into a Kohaku of their own.
Momoclo's actions I find awesome and historically courageous considering way back when, when Pink Lady turned down an invitation to the Kohaku at the pinnacle of their success, reporting that they'd be creating their own New Year's live show. NHK was such a power house then and insulted that a couple of broads would slap them in the face like that, summarily drummed them out of show business in a tsunami of shame. Pink Lady much later regrouped, but until Momoclo nobody ever dared making their own New Year's show in the shadow of NHK.
