The quality of the mix has changed due to the inputs. No natural virgin asphalt is used now, less refined asphalt is used, more recycled asphalt is used. Recycled asphalt has been exposed to more UV and oxygen leading to altered chemical composition to various degrees. This isn't necessarily a bad thing however a changed variable you must consider. Also asphalt is inherently a malleable product that is only as stable as the base upon which it is laid. Few things get the base preparation of projects in years past.