It just seemed very intuitive given the presence of top color option.
Hacker News - a site where people make a sport of middlebrow dismissals of others' work and complaining about the features of other sites and software they very likely have never had a single PR accepted for.
The real answer is that PG probably just added to topbar feature to have a karma reward for users to work towards as a means of positive reinforcement (I haven't actually found a specific quote from him on that) and also that the current admins tend to be conservative about adding new features, so likely they don't believe a dark mode would improve the quality of the site enough to be worth the effort.
Rather than point out all the things wrong with this question, I'll leave it as an exercise for the reader.
I just asked because there is an option to change the top bar. It seems weird to not have dark mode given the nature of the users on the site but I would understand if there was an intentional reason as there are in many tiny things here.