Because the Mediator 1200 doesn't have any PCI slots.
OMFG. The card I linked to is a Zorro card, not a PCI card. The suggestion I posed, and question at the same time, is "
Would a Zorro slot FastATA board designed for A3000/A4000 work in a Mediator?" IF SO, it would eliminate OP's problem of having to cram an A1200-version FastATA card in-between their Mediator, Indivision, and other expansions. But I don't know if the Zorro-based version of the FastATA will work in the Mediator 1200. Not because It's PCI, but because it was designed for A3000/A4000.
http://amiga.resource.cx/exp/fastata4000For chrissake, I give up. Too many facepalms in this forum, today.
Edit: Nevermind. The Mediator 1200 doesn't have any Zorro slots? Just googled a picture. What a piece of crap. I just assumed that every Amiga expansion busboard included at least a few Zorro slots. Meh.