That Schumer and Gillibrand are not campaigning for Hochul is a sign of her toxicity. Simply put, there's nothing in it for the senators. If Hochul wins, it's business as usual for the senators. If she loses, then they appear weak for campaigning for her in the loss.

Considering that she is basically tied with Zeldin in a state that has 2-1 Democrats to Republicans shows how terrible a candidate she is and the senators want nothing to do with her possible loss.

As for Hillary, well, she's still lusting after a position she lost twice for, so she has some reason for burnishing her campaign credentials. If Hochul wins, then Hillary can claim some handiwork in the result (whether that is true or not doesn't matter since it's the perception of such that matters). And Kamala, well, sending her is Biden's way of saying I want nothing to do with Hochul possibly losing.

And as for Hillary's popularity, I doubt it was ever all that much. I mean it's not Michelle Obama level. Sure, Hillary won NYS Senate seat in 2000 and 2004, and won NY in 2016, but that's a span of 22 years to six years ago. Times are different now.