Using the Force requires you to gain mental focus, ideally by meditating, but in a pinch (such as during a fight) by simply concentrating quite hard. By going on the offensive constantly, Grievous was able to rattle Kenobi sufficiently that he wasn't able to use his Force powers. Note that as soon as Obi-Wan is able to get a little physical distance, he's able to use a Force push.
As to why, specifically he didn't just flip Grievous' lightsabers off, that would seem to require a level of concentration and precision that needs far more than a second or two of focus. Attempting such an intricate move in the middle of a fight would likely result in his immediate death, and that's assuming that Grievous doesn't just beat him to death with the handles.
It was almost like fighting four different people at once. Obi-Wan’s
lightsaber blurred as he blocked and parried, but he knew he couldn’t
keep that up for long. Time for a different approach. He leaped,
flipping high over Grievous to land behind him.
Grievous didn’t have to turn; he just rotated his mechanical body
until he faced the other way. But even doing that took time and threw
off his attack, just enough to let Obi-Wan’s lightsaber swirl past his
guard. Two of his four arms dropped to the floor, still gripping their
Before Grievous could adjust, and attack with his two remaining
lightsabers, Obi-Wan reached for the Force. He lifted Grievous into
the air, throwing him against one of the beams that supported the
upper level. The impact shook the lightsabers out of his grasp. They
landed on the floor of the control center, while Grievous slid past
the edge of the floor and fell to the level below.
SW: Revenge of the Sith - Junior Novelisation