They have an empty bench.
So, the Democrats decided that it would be Hillary's turn in 2016...so there were no serious contenders. In the past, it has been the sitting Vice President who was the heir apparent. Biden was chosen because he wouldn't be a contender in 2016.
So, for the past 16 years, there hasn't been any up and coming Democrat presidential contenders because the Liberals were all waiting for Hillary.
They are now realizing that Hillary's time has passed (if there ever was a time!), and they are realizing that they are SOL.
The leadership of the Democrat party is made up of aging White liberals and young radical minorities. The party is in trouble.