Probably a combination of reasons.
- A large portion of the posts are for startups/small companies where senior positions are too expensive or not really needed yet
- A lot of companies prefer to fill senior positions via internal promotions/network hires rather than external postings
- There tend to be relatively few open senior positions to start with at a lot of companies. Competition for these roles is ridiculously intense, so they fill really quickly compared to lower level dev/engineer roles
- There's a (justified in my opinion) perception that most of the HN crowd is more suited to dev/engineer hiring than it is to more senior hiring, so the postings are going to naturally reflect that skew
In a similar vein, you could ask why Who's Hiring has so few posted positions for non-dev technical roles beyond entry-level product managers. The reasons are pretty similar to the above.