Because the hardware didn’t support it. We knew in 1999 that the 2038 problem existed, for example, but our choices were a) stop representing dates as seconds since the Unix epoch, and introduce who knows how many bugs in every imaginable piece of software, or b) kick the can down the road until 2038, when presumably everything would support 64-bit integers. b) was the obvious choice then, and it still looks like the right choice to me in hindsight.