There are a number of causes:
1. Most likely, the cables/housing have corroded and the excess friction is keeping the brake from returning properly.
2. The brake pivots could be dirty/corroded causing excess friction.
3. The brake could be assembled too tightly.
4. The brake return springs could be broken/weak.
A good way to start determining the cause of the problem is to disconnect the brake cable and close the caliper by hand. Does it spring back out or not? If it does, the problem lies in the brake cable/housing. If it does not, the caliper itself is the problem.