Why are proteins named so?

In Greek, proteus means that which is of first importance.

Out of the major three components in diet (carbohydrates, fat and proteins), it is proteins which build the body and form enzymes and hormones. A deficiency of carbohydrates may be made up by fats and vice versa, but a deficiency of proteins cannot be compensated by any of the other dietary components.

So, of all the dietary components, proteins must get the first priority. That's why they are named so.