Because I have a long-running feud with the darktable team and I don’t want their stinky users and developers flooding the forums*. 
Or, more like what @houz and @paperdigits already said. Though, @Morgan_Hardwood was asking the other day about creating further categories to delineate software discussions further, which I suppose we could do if we wanted an easy way to filter for darktable specific discussions. They’d still show up in the main Software category the same as the others.
* Please note that I do not in any way intend to disparage all of the fine, stinky people using darktable. I love them all and would even willingly share a flat again with the developers. 