Low Kick (and most other Fighting moves) > Brick Break all day, every day.
Hence why you see Low Kick Kangaskhan and not Brick Break Kangaskhan, Drain Punch Conkeldurr instead of Brick Break Conkeldurr, Superpower Landorus instead of Brick Break Landorus, etc. Low Kick really only hits Tyranitar, Excadrill, Kangaskhan, Heatran and Bisharp. It hits all of those harder than Brick Break does. Besides, Screens are almost nonexistant in VGC and Low Kick will usually be more useful.