Re: UV/IR model. Making that change is relatively straightforward; companies like Kolari Vision offer it as an aftermarket service. In contrast, the color filter array seems to be more tightly integrated into the sensor itself. So, a monochrome camera requires spec'ing a whole new sensor rather than just slightly modifying the manufacturing process.
Re: benefits of monochrome sensor. To me, it comes down to the character of the noise, which is always an issue at high ISO but can also be an issue at low ISO with significant processing (such as is almost always the case with high dynamic range scenes, aka bright daylight). Let's simplify things and assume that "noise" means that an arbitrary sensel incorrectly reports a value of 255 rather than the tone of the actual scene. When the image is derived from interpolated values from neighboring sensels, this means that at least three other pixels are affected in a Bayer filter, with more than that likely in an X-Trans filter, but no other pixels are affected in a monochrome camera. At this level, then, a 24 megapixel Bayer camera could be meaningfully compared to a 6 megapixel monochrome camera. For some applications, this is not a meaningful advantage; for others, it makes all the difference.
In brief: monochrom sensors yield images that have finer details and smoother fields than color sensors of comparable technology and at the same image resolution and pixel pitch.
Why hasn't Fuji offered a monochrome camera? Well, I'd guess it is because the trade-offs are difficult to understand, and forcing consumers to choose between products without clear differentiators is a recipe for marketing failure. Offering a monochrome camera is tantamount to admitting that the color camera's B&W images are inferior, which they are even if they remain excellent, but which never sounds good to a prospective customer. Moreover, a monochrome camera sacrifices the possibility of correcting lateral chromatic aberrations in software, not to mention emulating the color responses of different filmstocks. So there are engineering, finance, marketing, and customer satisfaction concerns.
Leica and Phase can get away with it because they are already far enough up the ladder of diminishing returns that customers are expected to understand the hardships they'll endure to achieve the other benefits of those systems. Fuji is far more mass-market and therefore needs to be more pragmatic in its designs -- although both the dwindling/upmarket-trending dedicated camera market and the GFX system might open doors here.
At least, that's my $0.02.
Cheers,
Jon
