Regardless of intent, it's unclear to me why one is valid and other other is not. If someone could point to the relevant part of the language spec, that would be especially helpful.
class Foo {
bar() {}
}
Foo.prototype.baz = function() {};
const f = new Foo();
new f.baz(); // Valid
new f.bar(); // invalid, throws errorAs pointed out in the comments on the original question, the spec defines it this way. A constructor must have the internal [[Construct]]s, and while ordinary function expressions do get this slot set, method definitions do not. This is described in MDN (as another commenter pointed out) as well.