I was thinking about this since I am interested. Here’s my “SPECULATION”. The issue could very well be size/spacing. The chassis is the same as the Mazda3 (compact), which has a bench that goes door to door. The Mz5 has captain’s chairs in the middle, which are kinda wide. Add in a healthy/comfy gap between the door and the outer arm rest, you loose a good amount of seating surface area. There really isn’t much space in the middle for a full sized adult. Having a middle seat would mean cramming 3 people into a even smaller space. I can image possible heads knocking (American sized passengers) in T-bone collision.
That said, if it is cheap enough ($300-$500) I may still consider for other purposes but it sounds like it won't fit previous gen. Also, since the OP didn't mention a price it's probably too much -for me.