Probably because GPS units are cost out of company's budget while car thefts are covered by the insurance policy.

So a GPS unit for an entire fleet of cars add up. Then there's the tracking (who's doing the tracking? Who's in charge of the tracker repositories, how do they coordinate with cops), serialzing etc. Which is a logisotical nitemare. Probably easier to claim insurance. Now if it happens often enough that the premiums are going up by a decent margin that's another story.

Edit: this was very incorrect regarding the insurance coverage reasoning.