They (The Democratic party) have been able to beat back challengers because their selection process for the presidential candidate is ironically less democratic than the Republicans. The GOP super delegates cannot vote against the way their state voted and they have less influence. The DNC super delegates have a greater influence and do not have to vote inline with their state.

As for Sanders. He has lost influence because he betrayed his base twice (he endorsed a candidate that the base consider to be totally corrupt) and lets face it he is too old now.