Final Fantasy is NOT a long-time PS exclusive series. It started out on the NES, and has games on the SNES, GBA, DS, PSX, PS2, PSP, PC, and 360. The majority of FF games released in fact are not even on Sony systems.
Also, please go and look up what the word 'monopoly' means. Because the way you're using it is just plain wrong.
Halo is a first-party exclusive. It's owned and made by Microsoft. Final Fantasy is a third party game, made by Square-Enix. They have the right to release their games on whatever systems they want to, and will release their games where they think it will make them the most money. Same with every other game that has been released on both platforms. They're looking to make a profit and have reason (that is supported by development costs and sales trends) to believe that they will not make as much of a profit from only releasing on the PS3.
And last of all...this does not warrant its own thread. We have a thread for current console discussions. I know you know where it is.