Or the limits of the chip and systems of the era.
The PC was started being designed in an era when 4k was in common use, and was released with 16k as the entry level point when the hardware was ready. It used the 8088 chip, which could address a max of 1MB, and IBM needed room to map the BIOS, video, etc. into the upper areas of the memory map, which was common in that era before virtual memory mapping chips became defacto. That is the 640k dividing line, at a point of 40 times the memory that the base unit shipped at. Even if they had it higher, the BIOS and slots still needed to be mapped in somewhere in the 1MB space that was the max the 8088 could address.
The Apple II had similar design constraints from an earlier time, but nobody goes on about the 48k limit of its design until they did some bank switching on the ][+ to get that extra 16kb of RAM or more on the //e and //c.
XMS 2.0 wasn't done until 1988, (seven years after the PC was released).
Even when PCs were shipping with 80286's, which could address a whopping 16MB of RAM, most PCs of the time didn't have more than 1MB of memory.
By the time the 80386 came out, and things could address lots of memory, it was the software lagging behind, with most people insisting on running MS-DOS of the era of 16k machines.