Why are only magnesium and bromine currently extracted commercially from ocean water?

The correct reasoning for this choice lies in the practicalities of mineral extraction. Magnesium and bromine are among the minerals that are more economically viable to extract from ocean water due to their concentration levels and the existing technologies designed specifically for this purpose. The extraction processes for these minerals have been developed to a point where they are cost-effective, given their abundance in seawater compared to other minerals.

Other minerals found in seawater may be present in lower concentrations or may require more complex or expensive extraction methods compared to obtaining these minerals from traditional land-based sources, such as mines. This makes the commercial extraction of magnesium and bromine more feasible and advantageous over other elements, leading to the current situation where only these two are extracted on a commercial scale from ocean water.