Why do baby's get dehydrated?

Small volume. Babies get dehydrated because simply they are so small. Fever causes evaporation, and babies are all surface area comparatively. Thus a moderate fever causes decent water loss. Couple that with vomiting and some diarrhea and a lot of fluid loss happens quickly. Babies cannot be reasoned with that they need to "just drink more", so all that water loss leads to irritability, not thirst.
Not enough water in, Too much out, or both. Can happen with diarrhea, vomiting, diabetes, overheating, overexertion, fever, or just plain not drinking enough.