In my world, the planet (which is not Earth, but is suitably Earth-like) has been besieged by deadly monsters for about 1000 years. Their initial attack lead to an apocalyptic scenario which took humanity hundreds of years to fully recover from. Since then, humanity has banded into one large empire/federation and another apocalypse has been averted, but it requires periodic military excursions into the wild to prevent any of these monsters from entering city limits.
The most dangerous variants of these monsters are the ones that dwell in the sea. To combat them, part mechanical part biological mechs were built. They are 15 ft tall and fight alongside small and medium-sized submarines. They are very effective in their role, but are not used in land-based operations. Instead, soldiers either fight with something like the exoskeleton from the movie Elysium (Here: or just have simple ballistic vests that protect them from possible friendly fire.
My question is: why would these mechs be limited to water combat if they have been so effective in fighting monsters?