[Throwawayaerlei]

IBM's promises, largely delivered to this day, that their System/360 computer hardware and future generations would be compatible, as was that line of computers except for the lowest end special case? All the effort IBM put into emulation of their older machines in the microcoded models (all but two high end ones)?

Some time before then it was realized we had a "software crisis" as outsiders put it, and IBM realized they were spending too much money supporting many different designs, some evolutionary with minor changes?

Many ideas behind systems like Linux go back to the 1960s as well, see Multics, the primary inspiration for UNIX but ultimately a closed source dead end.

For the original question we have to realize how very small many old computers were. UNIX started on small DEC minicomputers, it would have made almost no sense at all back then to allocate more than 32 bits when your maximum data address space was 56KiB and your CPU was 16 bit (the PDP-11 family where UNIX became big following an incompatible DEC predecessor).