The short version of it is as follows:
1. The dems relied on high turnover to keep the ground game going, under mid-level staffers with actual managerial and leadership skills.
2. This leadership has been suffering a slow bleed for years, as individual members of it retire, die, or fuck off in disgust.
3. The people replacing the rank-and-file, and in turn, the leaders, have increasingly been activists who have minimal ambition and ergo, won't threaten the power structure too much if they go off the reservation too much (a legit concern with some of the newer ones).
4. As result of 2-3, the sort of necessary ground game leadership the Dems need is in critically short supply, and getting worse.
5. They have no solution to this. They chased off the oldsters by failing to deliver, chased off the moderates with abandoning them in favor of embracing the hard left types, and they chased off the talented people from who were left by only promoting suck-ups who couldn't threaten their carefully-stacked deck of cards.