Why should we store the restriction enzyme at -20ºC?

Question

anyone can tell me the reason why?

Answer 1

It prevents denaturation, aka enzyme degradation. Heat kills protein linkages.  

Answer 2

Not just restriction enzymes are supposed to be stored at freezing temperatures, many other enzymes indicate that they're required to be kept at -20ºC, like the DNA polymerase.

Putting the enzymes at freezing temperatures helps to lower the levels of energy that surrounds it, which equals to a diminished level of movement (because you know that proteins are moving, fluid entities), and by doing so, you help to protect their tri-dimentional structure, which is the one thing that enable the proteins to be named enzymes