Why would manufacturers use Ni-MH batteries in a robot vacuum cleaner?
You forget about other factors.
capacity/volume/weight ratio's and price.


Since it's a robot, discharge is controlled and not usually influenced by the user.
NiMh for a given capacity is cheaper
Higher weight is actually an advantage for this type of product.
Less electronics for protection and possible liability issues for the manufacturer or distributor.


All these or a combination of several of these probably tip the scale.