They did not knew the route of the aircraft, and even if they did it shouldnt have helped them as the aircraft is Stealth (almost invisible to radar).
Actually yes, it would have helped to know the route. Stealth serves to shrink detection. If you know the route, you simply place a SAM close enough to said route.
They moved to Simanovci, their standard wartime firing position (an old SA-75M Dvina [SA-2F] position), and simply sit there.
(not moved under any route)
The real cause was just what I written earlier.USAF over self-confidence in Stealth technology, till 1999.
They let to fly the four Vega plane into Serbia, without any support.
No F-15C, no F-16CJ, no EA-6B, because of weather reasons...
... those poor Falcons might have been rusted.
Without Weasel threat, the SAM's could operate freely...