How do they think Sanders has such a low chance?
Because sanders has a very low chance of winning.
Here's the thing - the internet is very, very easy to make into a giant echo chamber where you only hear people saying things you agree with. Sanders is incredibly popular with 18 to 20-something white, middle class college students, who also happen to dominate social media. So it feels like sanders is neck and neck with clinton, because that's all you hear, echoed back at you over and over. The media helps fuel this because "queen clinton still only democratic candidate" makes really poor news so they play sanders up to
sell papers draw those ad clicks.
However, in reality, he's doomed. He needed to win iowa by a landslide (or at least, by a very healthy margin) and he didn't, he lost.
Why? Sanders polls very, very poorly with minorities (especially blacks). I'm not sure what the current numbers are, but at one point it was 79% of black democrats supported clinton, and 19% supported sanders. Why does this matter? Because iowa is the 3rd whitest state in the country (~90% white). It's one of the most demographically favorable states for him and he still lost, what does that say about his chances in less favorable states?
He's probably going to win new hampshire (2nd whitest state). He has a chance in nevada (though not a very good one). Then he's going to get crushed in south carolina (over 50% of democratic voters in s. carolina are black) and then comes super tuesday when most of the rest of the country (which is significantly more racially diverse than iowa) votes.
So he's got less than month to swing a large percent of the black vote his way. Impossible? no. However it's not very likely, hence his low chance.