The tracking hardware is expensive both in terms of initial outlay and maintenance, relative to the increase in output (which is significant: ~30% or so), when compared with the cost of just adding more panel area. Tracking is useful if you are constrained by the area of panels you have or the area you have to put the panels, but if you are constrained by neither and just optimising for price they lose to just building more fixed panels (in fact, with the rate at which panels are getting cheaper there's even a bit of a push towards not even bothering with building the mounts to orient the panels in the best fixed orientation and just putting them flat on the ground).