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.
There are millions or billions of quadrupeds surviving on 3 legs every day. 4 legs is evolutionarily superior to 3 but 3 gets the job done, especially if you don't have to worry about agility or repair.
If the robot has two legs with wheels at the ends it could combine speed and flexibility. Adding a third leg for additional stability when the robot needs it might be great. But the third leg would probably need to be between the first two and I don't think humans are mature enough to have that.
I agree on the rest but don't see why it would be heavier. If you have more feet standing at any particular moment the load would be more distributed, i.e. movement should be easier with more legs (depending on leg weight of course).
Especially because this thing is already $8k, I imagine they have already done some substantial price optimization.