Members of SNH48 Team X and Team NII declined offer to participate in Girls Planet 999

Quote from bartkun

i found online that SNH48 planned to send six girls to Girls Planet 999 but apparently members of Team X and Team NII declined offers. So they will only send 4 members from other teams.

I think they've picked from popular members with busy schedules and girls just didn't want to give up of all promotions that they were currently doing in China. This even fits rumor that Song Xinran was supposed to be in Girls Planet 999, but girl is busy with cf's and filming cdrama so she probably didn't want to ruin her "moment" in China.

Yes, there are also quarantining issues. Two weeks after entering Korea and two weeks are returning to China. In between, a few months trapped in a hotel or resort for the show.