JavaScriptのprototype

function person(name){
  this.name = name;
  this.prof = function(){
    console.log("name : " + this.name + "\nage : " + this.age + "\ncolor : "+ this.color + "\nweight : " + this.weight);
  }
}
person.prototype.color = "red";
person.prototype.age = "18";

var man = new person("Jackbow");
man.weight = 50;
man.prof();

---

この man.weight = 50; を

man = {
  weight : 50
};

にしてはいけないのはなぜですか？