Keccak and Poseidon are both sponge constructions. Keccak’s permutation function is uniquely invertible. This simplifies and strengthens security arguments. Keccak hides 256 bits of internal state when producing an output, so as long as the permutation is chaotic, Keccak is secure.
Is Poseidon’s permutation function uniquely invertible? Can you find two different internal state inputs that permute to produce the same internal state output?