alexban, i read your question with an open mind and i think it is a good question. you know what your ops manual states but you don't know WHY it is much lower than that of a manual landing, and i guess you are wondering what would happen if an autoland was attempted with a much higher crosswind.
to get straight to the point....a manual landing involves the pilot decrabbing with the rudder. there is no rudder channel in the 'stock' 737 autopilot thence the autopilot cannot kick off drift. the aircraft is capable of handling a certain amount of side load on the gear but obviously boeing have decided that 20kts across covers it nicely (that equates to 8 or 9 degrees of crab at touchdown). I believe there is a three channel option but I havn't flown it so know nothing of it's performance.
From a commercial point of view, the vast majority of low vis landings are in foggy conditions with little or no wind. I think it is reasonable to assume that boeing have designed the aircraft to cover this sector of the market. the extra cost of having a rudder channel with runway alignment software in the FCC's has been weighted against keeping the sale price down. the technology is there but boeing made the decision which way to go based on market forces and we have to operate the aircraft based on that.
personnally i wouldn't like to land in a sandstorm with a 35 kt crosswind in 200m visability and be handed control by the autopilot at touchdown (ops manual says disconnect as soon as possible after touchdown) with 15 degs of crab on and then try find the centerline. if the drift had been kicked off by a rudder channel this would be a whole lot easier ( like the md11/tristar/767 etc).
i wonder did all the guys/gals who took the pizz out of your question know that!!!
