Because the sensors under the street can’t detect your two-wheeler.

Why do they always say
"because motorcycles and bikes aren’t heavy enough or don’t have enough metal to trip the sensor"
instead of
"because traffic engineers in this city don’t design and adjust the signals to detect small vehicles"?
– sggoodri on Bike Forums

https://www.bikeforums.net/showthread.php/723386-New-Law-for-Bicycles-stopped-at-red-lights?p=12424477&viewfull=1#post12424477oldId.20110329100344443