While the mechanism of the kick is almost the same in both kicks, the target is different. The thigh kick aims at the end side of the quadriceps, a couple of inches above the knee, while the calf kick, as the name suggests, targets the side of the calf and the area below the calf.
The sciatic nerve has its root right in the side of the calf without any muscle over to protect it, once you hit the nerve, the limping begins immediately.

We have seen this in MMA and Muay Thai over and over again, it takes only 2 or 3 clean and stiff kicks to the sciatic nerve and the fighter is crippled instantly.