Yeah having different rules for retrofitting is understandable (Although it would be good if those who retrofit were at least required to improve the status quo

Yeah having different rules for retrofitting is understandable (Although it would be good if those who retrofit were at least required to improve the status quo of the building, if not required to bring up to the standard of new building). But what blocks the code from saying e.g. U=1.2 for new construction which is a normal (and honestly pretty cheap) triple glass window?