Phantom Force has lower base power (90 compared to 120). Shadow Force is also Giratina's signature move.
As for why they made the move, only Game Freak know for sure, but I will list a couple of theories.
- They wanted to keep Shadow Force as Giratina's signature move.
- They wanted to use a move like Shadow Force, but they thought that Shadow Force was too powerful so they made another move that was a bit weaker.