I THINK that the reason is: 8086 inherited it from the 8085 which inherited it from the 8080. The next parent in line would be the 8008, but that has a small call stack in the CPU registers rather than RAM, so the ancestor would be the 8080.

The 8080 had 64KB address space. I bet the rationale was to partition memory so that classic memory usage goes upwards from 0000 to FFFF and the stack goes downwards from FFFF to 0000. This removes the need to define a boundary between the two beforehand.

Of course this is totally speculation on my part, I might be super wrong.