Thigh rigs suck for normal patrol work. They offer more difficult weapon retention, slower draw, and less comfort when seated in the car.
Thigh holsters are a compromise for soldiers/tac team officers wearing large external body armor that interferes with their belt line. These officers also carry a long gun as a primary, so the pistol is not as important, and is backup to the long gun.
Somewhere along the line people thought thigh holsters looked cool, so people started wearing them. Their is a female officer in my jurisdiction that carries one on normal patrol and it looks stupid IMO.