The 8088 CPU requires that its reset vector be placed at the top of its 1MB address space [1]. So even if the IBM engineers had wanted to put all the reserved

The 8088 CPU requires that its reset vector be placed at the top of its 1MB address space [1]. So even if the IBM engineers had wanted to put all the reserved space at the bottom, there still would have been a tiny "reserved" hole right at the 1MB barrier due to the design of the chip. So with the requirement of the physical CPU hardware for a small reserved space at the top of the (at the time) maximum addressable range resulting in a non-linear memory map anyway, putting other reserved stuff (ROM, I/O memory, etc.) up there was not so much of a bad idea at the time. They clustered the "ROM stuff" up there along with the required reset vector. Also, remember, hindsight is 20/20. The IBM engineer's in Boca Raton, in 1979 to 1981 [2], designing what became the IBM-PC likely had no idea just how big of an industry wide impact their choices were going to eventually have. [1] https://en.wikipedia.org/wiki/Reset_vector https://en.wikipedia.org/wiki/Reset_vector [2] https://en.wikipedia.org/wiki/IBM_Personal_Computer https://en.wikipedia.org/wiki/IBM_Personal_Computer (released Aug. 12, 1981, so the design was created some length before that date, 1979-1981 is a guesstimate)