I'm only speculating, but the problem may be the complexity of the paint server interface. Using it requires FuncIRIs which-- as one commenter on another thread

I'm only speculating, but the problem may be the complexity of the paint server interface. Using it requires FuncIRIs which-- as one commenter on another thread pointed out-- can cause name collisions with inline SVGs. You could of course abstract away that problem with javascript post-processing-- but then you have yet more complexity, this time for a problem that doesn't exist in the domain of CSS. That limits the expressivity of the feature. For example, I wouldn't be able to just play around with SVG mesh gradients inside about:blank in Firefox due to a deeper problem with resolving FuncIRIs. Whereas I can trivially create a CSS gradient there and experiment with them.