The stoplight switch is probably defective. The cruise control cancels when the brake lights apply, and the crusie module uses feedback from the switch to make sure it is working before it engages. It could be as simple as the switch being unplugged, or the plug is loose.
There are a few other things it could be, but this is the easiest to fix, cheapest, and most likely thing.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.