It's all about compatibility..
If I remember correctly (and I never used Acorn machines), the disk format used was substantially different than on the PC.
There were, in fact, 1.8MB PC disks used in commercial products - most specifically by OS/2 Warp. XDFcopy had to used to create the disk and XDFLOPPY.FLT was the filter driver to read it.
Unfortunately whilst it worked on most drives, it did not work on absolutely everything and, IIRC, it was slightly harder to find disks that were 1.8MB capable rather than 1.44MB.
On a similar note, Microsoft once planned to do automatic floppy insertion detection - unfortunately two incompatible schemes were in use by the manufacturers. For the sake of compatibility they didn't bother.
I never did get around to using 2.88Mb disks