[briHass]

A good amount of modern building code is related to energy efficiency. These standards applying to new construction make sense: it's only a marginal increase in cost to add insulation, use upgraded products, or ensure air-tightness during construction. However, for existing homes, this would be extremely costly. The house would, in many cases, need to be ripped down to the studs on exterior walls and completely reinsulated/sealed, older windows would need to be replaced, HVAC ducts exposed and sealed/insulated, etc.

For an existing structure, the result of all these improvements, even considering rising energy costs, would never pay back over the expected life of the house. Code tries to strike a balance here by enforcing that when it's reasonable to do so (e.g. you tore down to the studs for a remodel anyway), you need to bring the areas modified up to modern-ish standards.