Because Apple's first screen for professional use since the Cinema Display shouldn't have been an ultra niche curiosity (or at least not JUST that). Shows how out of touch they are.
Apple primarily got criticized for putting 'Pro' behind products that didn't meet quality & innovation targets tech professionals expect & demand. Accordingly, I'm not sure it's out of touch this product existing and Apple finally tasking steps appeasing to such audience w/ the Mac Pro & Pro Display XDR.
The actual target audience of pro devices isn't complaining in masses with many freelancers at worst admit it's overkill for them but wouldn't mind their clients/employers paying for them which is exactly what these products account for with their design & price.
The 'pro' audience Apple is referring to is deliberately vague and has been conflated with a lot of professionals who aren't tech professionals in salary, responsibilities, & tech needs over the years.
There are also professionals in industries that always position these products as halo products for their employers to pay for, but never actually themselves (many small business photographers & developers + all tech writers come to mind).
Overall, the product isn't niche more than it has correctly positioned itself to be readily available for creative professionals checking most of their boxes that need or prefer MacOS (with the notable exception of *MacOS* not supporting Nvidia GPUs & AMD processors and Pro Display user-customizable reference modes coming soon rather than at launch), at price ranges only professionals can comfortably afford. It's well-positioned among the existing line-up of pro monitors offering great impossible-to-copy, hard-to-copy, or will-take-a-while-to-copy features for professionals (the closest competitor, the Asus PA32UCG, will not launch until Q2 2020).
This all makes sense considering the products were revealed at a *pro* conference, not their consumer-oriented events that usually take place in the Fall. It is at those events instead Apple will likely reveal consumer-level products like a low-end but just good enough monitor for average joes (a low-end monitor that would make consumers very happy seems imminent for the Mini Pro alone)