Going from 2 to 4 legs doubles the amount of actuators required and substantially increases power consumption since you must move more mass, going to 6 compounds the problem further. In a future where we have more dense power storage and better (and cheaper!) motors, you probably will see robots with more legs. But for now, the most efficient solutions are bipedal.

Especially because this thing is already $8k, I imagine they have already done some substantial price optimization.