Ahead of the question, little warning. Question is not about what are the better options. Question is why it is bad idea?
Ok, suppose we have plaintext $p$, which divided into blocks: $p_1,p_2,p_3,...,p_n$
CBC (cipher block chaining) can be then defined as $c_1=E_k(p_1 \oplus IV)$,$c_2=E_k(p_2 \oplus c_1)$,$c_{i+1}=E_k(p_{i+1}\oplus c_i)$
Ok. What would be bad about this version $c_{i+1}=E_k(p_{i+1}\oplus p_i)$?
($E_k$ is some encryption function)