....is 2 fold
1) Clinton fatigue (and we haven't even reached the primaries yet). All that Clinton baggage, that people are coming to be reminded about, once again. Let's put aside for the moment the utter incompotence as SoS, and the anemic lack of any accomplishments (no, titles are not an accomplishment, nor is traveling on the tax payer's dime), and people are reminded about all the slime, unethical conduct, and the ongoing glass castle they put themselves in, where rules are not to be applied to them.
Then add incompotence & anemic lack of accomplishments
2) Literally, the # of GOP candidates. The only means that Clinton can recover, is to turn the spotlight off her, and onto her opponent. Problem is, with all the GOP candidates, and no specific front runner to focus on, that's not possible. She's made vague references to Republicans in general, but it has no traction. She has to unleash her minions on someone specific, in order to get that Clinton machine going
So, until that happens, her poll #'s are going to continue to tank, while Bernie sits back and smiles bigger & bigger