Simply because there is no market for it and monochromatic cameras will always be more expensive due to that.
It does have benefits: more light, more resolution and higher QE%.
For the purists, more "pure" in a sense, too.
The downside is obvious. To get a colour image, you will need RGB filters. You need high-quality bandpass or even narrowband interference filters. These are often over $400 (Semrock) or $200 (optolong) a pop. For a set of three, you are looking at upwards of $1500. Mind you, these are only 25mm in diameter. Now imagine how much a 52mm one would cost. What about 77mm?
https://www.idex-hs.com/store/product-detail/af01_551_22_25/fl-433417?cat_id=semrock_optical_filters&node=individual_optical_filters Therefore, the only economically feasible option is to install it in front of the sensor.
Now you have introduced a ~10mm thick piece of glass. Whoops, you have to alter your lenses slightly to compensate for the extra path length, or else the lenses will be soft -- what is the point of monochromatic cameras again? Time to introduce a special class of lenses that nobody would buy.
Would you trust the user to install something right in front of the sensor? Time to increase the price to compensate for the extra RMAs due to stupidity.
So the conclusion is simply forget about colour images. Will you miss them? Yes, you will.
There is just no market. It is more feasible to convert a camera to monochrome. The bayer array can be scraped off with a bamboo chopstick.
These cameras are particularly useful for scientific imaging and deep space astrophotography.
For example, you can view JMC's extensive gallery of diatoms here:
https://diatomimaging.com/gallery1/He uses a BW converted Nikon D850 camera. I was told that he uses jpegs straight out of the camera.
I have some as well. These were captured with a Hamamatsu camera. The colour image was a blend of RGB stacks.


(I really wish we could get square sensors on normal cameras. Modern lenses can fill them well, and it will offer greater cropping capabilities.)
If you are interested in monochromatic conversions, a D5100 running Nikon Hacker firmware would be solid. I am thinking about potentially offering converted cameras for sale but the amount of pre/post-sales work puts me off.