They said it's because of the members' safety. I don't think it's the one and only reason. SKE48 also made a MV with all members participating even during pandemic outbreak. There might be money issues and they might have noticed, that it's important to include Nako & Sakura in the single to fix the money issues...
In the same time, the pandemic is currently storming Japan again, and more and more prefectures are placed under state of emergency. Fukuoka is no exception.
That's sad, finally HKT had a new release and now it's postponed ._. I wonder how long they'll delay it, it might take some months. I guess nothing was ready for the single (like the MV recording etc. maybe the senbatsu wasn't picked yet either) Because if it's postponed to after April, they might have to change everything, since Sakura and Nako will most likely return. Let's see what happens... I just hope we'll get the single in the first half of the year, and with a good song!