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.
That's a factor but they have also been more willing to coordinate to stop outside challengers, like when everyone (including the frontrunner!) stepped aside to make way for Biden.
Base aside, I think Sanders isn't influential because the party insiders are strongly opposed to him. By way of contrast, despite a disastrous primary run, look at Kamala Harris.
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.