Original: However, this comes at a cost: because the keystream is now pseudorandom, and not truly random, the proof of security associated with the one-time pad no longer holds: it is quite possible for a stream cipher to be completely insecure.
Simplified: However,  because the keystream is pseudorandom, and not truly random, the security associated with the one-time pad cannot be applied and it is quite possible for a stream cipher to be completely insecure.