NPR's political reporting is pathetically weak: it's all about the horserace, never about policy.
This motivated me to donate to his campaign again.
I'd imagine that there's some kind of grudge against Sanders in the media class, similar to the obsession with Russia.
The MSM was entirely confident in Clinton's win, and were reeling for answers after she lost. He was Clinton's main challenger in 2016, and I know a lot of liberals saw his campaign as somehow detracting from hers, so much so that it contributed to her losing the election.
We already know what it is, the centralized ownership of news media. Billionaires do not want Sanders to bring their taxes in line with reality. Thus the corrupt political elites and the corporate owned media marginalize him every chance they get.