I don't know his answer, but I think two things might have influenced him:

1. Obama winning showed it was possible to get someone in the Whitehouse that the establishment didn't expect. Regardless of how well he has or hasn't done fulfilling his promises...he DID become president, which taught a generation that it was possible.
2. The internet has had a profound effect on how information is shared, and at this point it's been about 15 years since it became widely available in many households and/or local libraries. 15 years is basically a generation, a person from birth to being a functional thinking being.

I'm in my 30s, basically the oldest a millennial can be. I've been on the internet my entire adulthood, but not my childhood. In my late 20s, I sort of withdrew from the internet socially...I didn't adapt as social platforms changed. I sort of poked forlornly at the remains of LiveJournal, but hadn't switched elsewhere. I just stopped reading and posting and interacting.

Then, maybe a year or two ago, I rejoined. I found Reddit, I found Tumblr, and I began reading and listening to what was up again.

Honestly? I was shocked as hell at how far opinions on a lot of these issues had changed. I was always liberal in my thinking, open to new ideas, but I always kind of expected to be an outlier in this. And suddenly it's like...where the fuck did all you interesting people come from? You weren't around when I was younger, or at least not in this form. There are really fascinating things being discussed by teenagers now that a few years back wouldn't have been discussed.

And thinking on it, I feel as if younger folks, in their 20s and teens, just kept on building and building on what was there before (the internet never forgets...), and it shifted how people felt over time. There's been a shift in opinions slowly happening for 15 years, and I think that's what Sanders saw, and why Sanders realized he might actually have a shot .

Popular opinion on all of his positions has to be extremely, extremely different from what he saw decades ago, especially since I'm far younger and even I've seen the shift myself just by not doing much on the internet for a year or two. It's maybe not noticeable if you're still a teen or in your early twenties, but this shift is very noticeable to me.

I don't think he would have had a chance 10 years ago. But he does now.

And having been out of the loop once, I now know it can happen again and opinions in society will continue to change as they want to, with or without me. (Something I think every adult should learn to account for--really learn, in their gut.)