I think your dad's case is simply a matter of bad luck. I have
met cab drivers (albeit, a few) who were willing to go the extra
mile and helped me with the luggage and etc.
As for the whole line of reluctant cab drivers who showed
indifference to your dad, all I can say is that a contributing
factor arises from our asian mentality, thats perhaps unique to
Singapore, of not breaking the norms and preserving the status quo.
Upon seeing the reluctance of the first cabby driver to help out,
the second, third and so on sub-consciously chooses not to
volunteer himself and stand out above the rest. Its the same
mentality that gives rise to the kiasu, kiasee and "scared of
loosing face".