The Democratic Party's establishment has done a much better job of holding onto power and controlling the party, compared to the GOP. Thus, if you are a Democrat looking for power, you aren't going to get anywhere unless you obey your marching orders from the usual suspects (Clinton juggernaut, Obama, Pelosi, Schumer, etc).
The fact that the party as a whole took a very hard turn to the left during the Obama administration also meant that fewer people who considered themselves very liberal would even think about opposing the establishment. You aren't going to get an upheaval without their support, and most hard core liberals actually thought that they were quite happy with the time under Obama and later, Biden.
Regarding David Hogg,
The DNC was hoping that he'd play ball, and support the establishment. After all, he was their darling puppet, having been pushed through Harvard (he had a "no-fail" tag on him), and most of the mainstream media's talking heads kept heaping massive amounts of praise on his non-worthy achievements.
The problem is that David Hogg thinks that he can buck the establishment, and turn the Democratic Party even further to the left. He was looking to throw his (light) weight behind the hard core left wingers that make AOC look moderate.
Hogg's hubris basically ensured that he would fall flat on his face, and this is but the beginning.
Also, his pocketing money from the DNC to fund his own PAC (aimed at overthrowing the establishment), raised more than just a few eyebrows. I can all but guarantee that anyone who does this isn't going to be thought of in a positive light by Hillary Clinton, and she's probably the last person that any Democrat would want to piss off, since the Clintons still wield significant power within the party, and unpleasantries tend to follow those who can or will cause trouble to them.