Because,
Final Fantasy have a Great Battle System,
I Think this games Look a Like Final Fantasy XII at Battle System, but about Graphic is more Better than Final Fantassy XII,
when I Play Final Fantasy XIII.. Like umm, Stay in Fantasy World.

Beside that,
Final Fantasy XIII so awesome at Character Skill and,
have many Monster at that World. Cool Special Effect!

Actually I Like their Soundtrack to.. so Beautiful Sound :'D