One interpretetation is that there's a lot more posting going on and it's hard for a single post to maintain a top spot as well as potential changes to the ranking alogrithm. Another option may be less engaged users. Maybe a mix of both.
I doubt it's that users are less engaged. It appears that HN had its most active Sunday ever today in terms of unique visitors. I've been meaning to look at some graphs of number of comments over time because it feels like those have gone up lately, too. If anybody else wants to make them, the data's public...
The other thing to consider is that no one of Steve Jobs's stature in the tech community has died since Steve Jobs. Or at least, died tragically. I hope Bill Gates (and his philanthropy) go on for many more decades, but I imagine his passing would be on notable to the tech community on the same scale as Jobs's.
Although maybe I'm completely overlooking someone who has died since Jobs, I would think the top deaths in each year would be among the top posts (e.g. Leonard Nimoy in 2015). My second most up-voted submission is Ray Bradbury's death...my top submission being a major drop in Linkedin stock price...Linkedin/MongoDB bashing is about as timeless as death :)