Agree with Coombs that it's about consistency. Personally I think it's the long consistent records of Corbyn and Sanders that made it possible for a broader group to get behind them.
If this is the start of the end for Sanders' campaign, it'll be a big task for the DSA to find someone with Sanders' appeal to push forward their rather orthodox social democratic agenda from here. People are not going to mobilise behind a 50 year old version of the same thing unless the candidate is exceptional.
The younger stars of socialism in the US like Ocasio-Cortez are not eligible to run and would meet with a reactionary response anyway.