Two possibilities.
1) They have to wait for Hillary! to dispose of Bernie!, before they indict and dispose of Hillary! and nominate their Dream Ticket of Biden/Warren.
2) They have decided to let Hillary! ruin herself in the general election, thereby completing the usurpation of the Clinton crime family by the Chicago crime family. The result that they envision is that Trump becomes President, then they pull every string available to them to wreck the country under his administration, and swoop in as the Saviors of Society in 2020. This presumes, of course, that Trump won't be able to handle all the planned country-wrecking.
I go with option two, but like the GOP establishment has found out, the Democrats will learn real quickly that Donald Trump is resilient as hell and he’s gonna be two steps ahead of everyone. The Democrats can run rings around the current batch of squishes in the GOP. That won’t be the case with Trump. He’ll be running rings around them too.