ground patrol cars or aerial surveillance require a person to operate them. GPS tracking allows for tracking many targets at once and then sifting through the data to find "bad guys".
the added costs of having a person track a target means more care is taken to avoid false positives. the person doing the tracking can also dis-regard irrelevant data that an automated system might erroneously use.
imagine some program that automatically sifted through a source of ubiquitous GPS data which actually did catch 100% of all terrorist attempts in New York in 2010 (the Times Square bombing attempt). what if that system meant 100 false positives were added to the no-fly-list because they drove on street X on date Y followed by street Z within 2 days (or some other such algorithm) when (if) someone asks about these no-fly-list additions they are simply told that the suspects exhibited behavior that was consistent with the Times Square bomber... if you are not a terrorist, you have nothing to hide!