Why are DB swings an appropriate tool for conditioning (breathing hard) and not KB swings?
I do not do KB swing to just breathe hard. If the goal is to just breathe hard do burpees and or sprints.
I use KBs to work hip extension in a dynamic, ballistic way. I place value on training the muscles in this area and nothing works this area better then KB swings using KBs.
I use specific tools for specific goals.