Why did Square do it?

[No.78]

FFIX was originally supposed to be a side game designed to act as a collaboration of elements from all the FF titles before it (the quintessential Final Fantasy, in other words). They later decided to give it a place in the actual series for whatever reason. The concept for FFX was already being worked on while FFIX was in production so no, they definitely never intended it to be the last FF game.

This is interesting because Crystal Chronicles uses alot of the same monster models as 9, and it is a tribute kind of game.