From SDS 27-51-00-002
"The TE flap UCM detection function uses data from the flap skew sensors and the flap lever position sensor. If the TE flaps move from their commanded position, the FSEU moves the bypass valve to the bypass position to stop the TE flaps."
One of the more important reasons is because there's only one Position Sensor per wing (2 total) and they're at the most outboard transmission. A broken torque tube as well as a defective transmitter would cause the system to believe the flaps are in position at all times.
The skew sensors (8) are mounted on all transmissions so that a flap segment moving out of position would be immediately detected regardless of the condition of the rest of the system.
(And boy if that isn't super-simplified I don't know what is...)