One of the major reasons for purchasing a particular derivative will, in many cases, be power consumption. The 'normal life' of a '51 is driving CMOS only and thus the requirements to drive is not much, which for the sake of power consumption allow a higher value of the internal pullup. Thus you see today two tendencies: the full size '51s get less and less drive capability (at the 1 level) and some of the smaller ones get more (see LED drive capability advertised).

Life is not a static event.

Erik