As to why there would not instead be a dedicated square sensor; for the same reasons that have been rehashed in innumerable threads about "why not a square format digital camera?": not nearly enough demand. And no, posts from many dozens of square format enthusiasts is not nearly sufficient evidence of commercially adequate demand. Instead, the total absence of square format sensors since the 36x36mm Kodak CCDs – even while the square format Rollei-based system was still around – is one big hint of low demand; the earlier clear move away from square formats back in the film era (when of course "square format chemical sensors" were readily available) is another.
Yes, that's an opìnion, but not a definitive one.
A 36mm x 36mm sensor isn't the same concept as a full 6x6 format negative which, if I remember, was in the 50-something mm squ. zone.
Using a 36mm squ. sensor would be delightful in a different, smaller than 500C body type. It could probably work very well.
That the Rollei didn't use a 36 squ. sensor isn't surprising - what would it have achieved - still too small to satisfy diehard 6x6 fans, and would just turn existing lenses into longer ones, which had that been the photographer's choice, he'd have bought already.
At least, that's how I see it at the moment.
Moving to 6x7 was something I did, first Bronica and then Pentax, and almost instantly regretted both times. My original Swedish Squares beat Japanese Rectangles any day - for me.
Rob C