I believe social activism has been associated with the Democratic Party recently, I suppose it is implied when you run under their umbrella.

It’s clearly the perception. Before Harris entered the race.

It doesn't need to specifically be Harris or Biden's policies to drag them down. There's very obviously a backlash against some progressive ideology going on, and the democratic party is clearly at least partly beholden to adherents of that ideology. That's why Harris can't give obvious and clear answers to (some) simple policy questions.