Your first portion, about the different between how everyone already knows a good deal about Jobs, but few knew about Zuckerberg, is a great point.
I just wanted to point out that the movie only covers from 1984 - 1998. The movie came out 17 years after the story it's telling ended. There's been plenty of time for reflection on this story. I don't think movies made in 10, 20 years will be able to do this better.
You nailed it right here. The reason the Social Network did so well was because it dealt with recent subject matter. It came out in 2010 at the height of Facebook's popularity and everyone was interested in learning about its origins and Mark Zuckerberg.
Conversely, Steve Jobs was released in 2015 and focuses on storylines from twenty and thirty years ago. I think it's reasonable to assume that your average Apple fan came on board initially with the iPod and then many more through the iPhone. But those products aren't given any focus so it's no wonder that today's Apple fan didn't flock to see it. If the movie had been about the behind-the-scenes work done on the iPhone, I bet it would've done exceptionally well.