For CAT 3b, you require rollout guidance in order to track the centreline after touchdown. For this, there needs to be an autopilot channel to the rudder. The 737 does not have this (although I understand it may be an option on the NG), whereas more recent types do. Certain failures in a 737 will leave you with one autopilot so you cannot Autoland. Similar failures in an Airbus will still leave you with two, although they may be downgraded. However, below 200' RA, you won't know there has been a failure until after you've landed (fail operational).