It's a few things.
- She wanted to stream on YouTube, which she can't do on Nazuna.
- She has been wanting to start breaking back out from her Twitcast bubble and return to regular streaming as Mikeneko. She had planned to do this last week before the unicorn drama made her take a break. She was supposed to do a special stream last Tuesday to kick off this comeback that ended up getting canceled. We still don't know what that stream was supposed to/will be.
- She's a bit down on Nazuna right now due to getting jerked around on a project she really wanted to do. She has been given the green light and then later told it was NG afterwards at least twice. It seems to have killed her motivation to do Nazuna streams this weekend.
She's also generally attached to the Mikeneko account and identity. Even if she acts mostly the same when streaming with her Mikeneko models as she does on Nazuna, she sees it as her actual identity instead of a character.
She's been streaming as Mikeneko on various platforms for over a decade and wants to continue growing despite everything that happened last year.
She has outright said she wants us to love her as a 3D real life woman more than any vtuber or character she plays/has played.