Mostly what others have already said:
- Outcast because of Kyle's self-imposed exile and abstention from the force following his little experience in MotS
- Jedi Knight II because MotS was effectively DF III, being a stand-alone game (requiring JK for validation but with separate engine code enabling it to exist on your hard drive without JK) and of a comparable (if not greater) length to the original DF
- Jedi Knight II also for marketing reasons: the JK name is more widely known in PC gaming circles than DF, so it is likely to grab more peoples' attention
- Jedi Knight II because in this game Kyle is a Jedi Knight as he was in JK (he was not in the original DF
- Similarly, not DF III because of the lack in this game of the "Dark Forces" (ie. dark troopers) which gave DF its name
That's about as many reasons as I can think up... for now!