Why do we get colds more than once in life?

Every time we get a viral infection (and survive), our body learns to defend itself from it. This is the basic principle of vaccines, and this is why we scarcely ever get any viral diseases (like chickenpox) more than once in life.

Cold seems to be the great exception to this rule, but it is not. Our bodies learn to defend from viruses which cause colds just like they do with all other viruses we meet along our lives. The problem is that colds can be caused by more than 200 different varieties of viruses. This means that every time we catch a cold, we are actually being victims of a cold caused by a different variety of those viruses.

Why?