I wonder how much of that is perception. Cute-utes are really popular right now as an entire class of vehicles while truly mini minivans are not. For instance the Kia Rondo didn't seem very popular either, even though both it and the Mazda 5 seem to pack a lot of functionality into a small package.
Mazda itself had very little interest in pushing the Mazda 5. It's own internal sales projections are very small. And it has *NEVER* been marketed here. Have you ever seen a commercial for it? Mazda in general is a low marketing company having really only marketed the CX-5/9 recently.
The 5 has a couple things that hurt it's sales a bit.
1) It's still a Minivan. Look at vans in general, they've lost A LOT of ground to SUV's. People have a serious ego issue buying a car with sliding doors as if those doors have circumcised their livelyhood or something.
2) MPG & Performance + size compared to full size. On paper the Mazda 5 really didn't get any better (if not even worse!) milage than a full sized Odyessy. Factor in a somewhat doggy 4 cylinder and a major step down in size and a lot of people just didn't want to bother with them.
Now if Mazda would take the time to "Skyactiv" the little bastard and give you a vehicle that could carry six people, was fun to drive, had the turning radius of a tricycle, and when needed storage space that would humiliate most cute utes....AND give you 35-40MPG on the highway...that would be something special.
As a previous Mazda5 owner, they are very cool little cars. I loved mine.