Now, why isn't the rear brake setting off the light?

 

The rear brake wasn't setting of the brake light because it was already lit up from the switch on the front brake not being depressed. The way the switch is set up, when the brake is off the switch is pushed in, when you pull the brake lever the switch is released and the brake light goes on. If the lever isn't lined up properly with the switch, it doesn't push it in at all, resulting in the brake light staying on. If you don't look closely it just looks like your tail light is on and your brake lights don't work.