There are a few reasons why it's not selling well.
1, Final Fantasy is declining in popularity.
2. Final Fantasy 7 Remake disappointed a lot of fans, so they won't bother with Rebirth. It sold 7 million in 3 years across 3 platforms, and not all 7 million are going to be returning customers.
3. Being exclusive to one platform limits sales. A PC and Xbox version day one would have helped a lot. Even if it comes out later on those platforms it's too late and sales will be low.
Square Enix (and Sony) will definitely re-evaluate if having Final Fantasy exclusive to PS is even worth it anymore.