Even if Rhythm Zone want to use Tokyo Kokusai Forum Hall A & Tokyo NHK Hall on 6th Dec , there will be events held at both venues
https://bachtrack.com/concert-listin...ber-2019/19-00
https://www.t-i-forum.co.jp/visitors...=2019&month=12
I'm surprised Zepp Tokyo was not considered cos out of all the Zepp venues , Zepp Tokyo capacity is the 2nd biggest
But same with Kokusai Forum Hall A & NHK Hall , there will be a concert taking place at Zepp Tokyo on 6th Dec
https://www.zepp.co.jp/hall/tokyo/sc...019&_m=12&_d=6
Same thing applies to Zepp Osaka Bayside too
https://www.zepp.co.jp/hall/osakabay...019&_m=12&_d=6
Events in Japan usually tends to get announced months if not a year in advance