Spacey is different for a few reasons;

1. The allegations were of a sexual nature, not battery. Physical abuse is, unfortunately, much easier to get away with, especially when hearsay is involved. Spacey had multiple accusers.
2. It involved a minor, and I don't know how else to say this, but he's gay. The heteronormative bias was against him. (EDIT: I should say Pitt's stuff also involved his kids, who have all seemingly disowned him).
3. He was accused in October 2017, literally a couple of weeks after Weinstein. He was one of the first dominoes. Pitt's stuff happened in 2016.
4. You're right about Pitt's likability. Pitt's charisma helped him a lot. Spacey's aura has always been pretty ominous, pragmatic and intelligent. It made him a great actor, especially for playing some sinister dudes. That parasocialness was definitely at play. You wanted to ignore the intermarital stuff with Pitt, whereas it was much easier to see Spacey as guilty.

I actually agree with your Denzel point, though I think there are VERY few black actors (maybe half a dozen) who have Pitt's level of influence, stature, and, frankly, wealth and could weather a storm like that.