Why were most of the metals used in antiquity class (b) (soft, in hsab terminology) metals?

The earliest days, it had to be soft metals, because the shape was made only through blunt force. The answer to this question relates to what forging techniques and knowledge were available to the given population at that time and place. As the most basic technique of hammering would be at the primitive end of the spectrum and it would also be the most readily available aspect, without any real guidelines or ability to judge the force needed to carry out the task, other than experience. Over time, forging techniques applied by heat and cooling in order to better mold and sculpt the metals accordingly. As well, some of the allows, or mixture of metals, which would come later and result in much stronger materials, could only happen because of advanced heat and pressure forging.