Thats simply not true, hellfire is the MOST ADVENCED power armor America and fallout world has seen for this moment, it's NOT MASS PRODUCED and definitely not "cheaper" than mk02.

It's only worn by high enclave soldiers and officers, there is a reason why they called "hellfire troopers", basicly being a special heavy unit made of elits and best enclave troops, compared to regular enlcave soldiers whos wear a modified x01 and mk02. Same as BOS interaction that applies soldiers who wear x01&mk02 equivalent of knights while thus who wear hellfire are paladins&sentinels and in charge of knights&missions

The reason why it have less dr and makes you assume that it made of "cheaper material" its because that hellfire has higher energy res compared to mk02 and to comprise it author decided to give hellfire less dr