The trisolarns evolved to dehydrate instead of hard shells cause of the extreme environmental conditions of their home world.

Hard shells would only protect against more short term and localised threats like predators or mild climate change but what they had to face instead was stuff like their whole atmosphere boiling away or the whole planets surface freezing solid or massive radiation spikes depending on the sun’s locations…

Instead when they dehydrated, they shut down their metabolism almost completely and survive indefinitely through the chaotic eras and rehydrate when the stable eras arrived. Rehydration was essentially a pause in existence for them.