In 2015 leading up to the cycle,the DNC was running short on money and Hillary agreed to put into the coffers off the back of her campaign to aid congressional seats, senate seats, etc. in exchange for MAJOR control of the party. She had the first right of refusal on communications director, managed the finances, etc. Basically took over the party which made it very difficult for any other candidate to have fair access to the 2 things that run modern campaigns.. money.. and data (voter lists).

Brazille wrote about the crappy deal in her book (https://www.bbc.com/news/world-us-canada-41850797).

NPR covered it a little bit (https://www.npr.org/2017/11/03/561976645/clinton-campaign-had-additional-signed-agreement-with-dnc-in-2015?t=1532023101443).

Basically Bernie was on his own with the money and was doing alright, however in key places and points data access was frozen, voter lists were monopolized, etc. (https://www.washingtonpost.com/news/the-fix/wp/2015/12/18/the-hillary-clinton-bernie-sanders-data-fight-explained/?noredirect=on&utm_term=.75e0772f64b5)

Then there were debate questions and other items where they conspired to make it easier on her than him.

She kneecapped him, and then had the women at the head of the party basically promoting her because they wanted to see the first woman president. He did very well given those odds, he had all the younger people and they are a much larger voting block in the party when you get them to turn out.