It did sell well however, selling over 3.5 million copies in the first 10 days. I don't know how high Squeenix's expectations were but it's wild they thought it would sell higher for a game that only released on the PS5.
The last numbered instalment, Final Fantasy 15, remains the series’ fastest-selling entry with 5 million units shipped and sold digitally in its first 24 hours alone. This was across multiple platforms, however.
Also, XV wasn't well received, generally speaking. I'm sure there are at least a few people (like me) who decided to wait on this one, to be safe. A damaged reputation can cause tangible sales impacts
When that only other platform was Xbox, though, I doubt that total remotely compared to PlayStation sales.
I honestly think it's just lack of trust in the series after 15 along with the economic times we're in, where people don't want to spend money on something they're not 100% confident in. It also didn't get the same media cycle as 15, and didn't have strong nostalgia value to coast on like 7 Remake.
I want you stop for a second and think about how long Final Fantasy has been around (NES) then think about how many other franchises from that time still exist. Then maybe try this comment again.