Each type of bathtub has its advantages and minuses, and a cast iron bathtub is perfect with its durability. Cast is heavy, but stable material with the longest lifespan. Even using a hammer you won’t make any crack on a bathtub from cast iron. Acrylic tubs are losers here. However, a cast tub is a very heavy stuff.
Even when it is empty it presses the floor (the ceiling of your neighbor). Consider the amount of water inside it plus your weight and the idea of installation a bathtub made of cast iron becomes even scary. What if you damage the floor with it? The owners of private homes who live on the ground floor are happy to decide themselves which bathtub they want. It does not matter which weight it has.
Moreover, they can buy such a unit for the garden or a back yard. When the heat is unbearable outside, it will be a fun to splash in the water and do not care if this water gets down. Your garden cast iron bathtub will stand on the ground. These units keep the water temperature perfect. they look nice in classic bathroom design with wall and floor tiles. Once you fill the tub with hot water, you can enjoy your bath for 20-30 minutes.
Cast can keep the water warmth for a longer time, but it is not good for health taking such a water procedure for so long. Considering the question of the tub maintenance, we should agree that enamel can be damaged easily, if you drop heavy objects inside it. The bottom can be restored, but the process is not easy.