Wouldn't this be because the ROM is taking up address space and they don't have bank switching? One of the future goals is to copy the ROM and RAM and switch it out of the address space.
Might have been interesting if they'd allowed you to page out the ROM to access the extra 16K - load a small routine from tape that contains your own tape loading routine, swap out the ROM, load the rest of your app, bob's your uncle. Would have needed some shenanigans to swap in the ROM on reset, obvs.
[0] http://www.breakintoprogram.co.uk/hardware/computers/zx-spec...
Long story short: it added 16K of RAM that was mapped at 0, and allowed both manual control with buttons or software control via a port to decide if it is mapped only for writes or it is mapped for both reads and writes.
https://microhobby.speccy.cz/mhf/103/MH103_20.jpg https://microhobby.speccy.cz/mhf/103/MH103_21.jpg https://microhobby.speccy.cz/mhf/103/MH103_22.jpg
https://microhobby.speccy.cz/mhf/104/MH104_22.jpg https://microhobby.speccy.cz/mhf/104/MH104_23.jpg https://microhobby.speccy.cz/mhf/104/MH104_24.jpg
> One of the future goals is to copy the ROM and RAM and switch it out of the address space.
Like the BBC Micro Sideways RAM system? Nice.