They are disabled on 32bit systems to save memory. We should have greyed out the option and had the mouseover say that, rather than just hiding the option. Pedestrians take up over 200 meg of memory and we didn't want to risk 32bit people being unable to finish their games because of memory issues.
Processor word size is not a good determinant of a machine's capabilities, my 32-bit laptop is much more powerful than my 64-bit one. Changing the default setting or decreasing the number of pedestrians is understandable, but I see no reason not to leave the choice up to the user. Omitting them entirely actually feels somewhat wrong, and removes quite a bit of fun factor from the gameplay experience: the world seems to be in an eternal cataclysm.
