The main issue seems to be that there is a far bigger overhead in designing and putting an active pixel CMOS sensor into production compared to a CCD, and the far lower sales volume of formats larger than 36x24mm mean that this investment would not be cost effective. Or at least so far, and in the judgement of potential customers like Phase One, Hasselblad, Pentax and Leica. I mention them, not the various possible sensor makers, because it is the willingness of these potential customers to guarantee some minimum order at some acceptable price that dictates whether a company like Teledyne-Dalsa goes ahead with such a low volume project.
I think of this like new models of commercial aircraft, where the decision to go ahead is based on whether enough deposits are put down by airlines. I am skeptical of the idea that sensor makers just design and produce specialty products like this "on spec" and then go looking for customers. For example, it seems that the direction of Sony's development of sensors for DSLRs has often been heavily influenced by the wishes of its dominant customer, Nikon, to the point that Nikon often gets to use a new Sony DSLR sensor before Sony's far smaller DSLR division does.
P. S. you cannot simply stick two sensors side-by-side to get something usable in a high quality MF camera. The resulting seam is tolerable for some uses like X-rays, and Teledyne-Dalsa does make some large CMOS sensors that way, but nor for "photographic" cameras. Instead, any sensor larger than about 33x26mm must be fabricated by "stitching", which means that it is "etched" onto the silicon part at a time, moving the stepper field between each part and being very careful with alignment, which lads to a high rate of rejects. Both Canon and Dalsa have described processes involving each piece being only 24x12mm. The Canon example involved a three piece stitch for the 1Ds sensor. As the size goes up and so the number of "joins" needed in this process goes up, the failure rate increases rapidly, which is a major reason why camera costs rise rapidly once the size goes beyond about the mainstream "APS-C" sizes.