Is it just perceived as being too uncomfortable for people to adjust their perception of someone they grew up thinking was a "good guy" in some sense?
I mean, as far as I could tell, everyone knew the ジャニーズ term and it was synonymous with these J-pop idols, but I never even realized there was an actual Johnny (I originally thought it was "Journeys" or just some nonsense brand name when I first heard it back around 2005).
When it came out after his death that Jimmy Savile was a child predator, he was excoriated and publicly denounced. Statues of him were torn down, and I think they stopped circulating his shows. He was the face of children's TV and philanthropy in the UK as far as I knew, and they burned him in effigy when his crimes were finally exposed. That's like if Mr. Rogers was credibly accused of committing terrible crimes; it would be hard to accept but when irrefutable evidence is presented you have to reframe your relationship with the celebrity you thought you knew.
Kitagawa isn't exactly the "face" of J-pop in that he wasn't a beloved artist himself... he's more like Harvey Weinstein: well-known and powerful in his industry, but although his name was attached to a lot of successful and beloved intellectual properties, no one considered them to be the real genius behind wholly responsible for this much-loved movie or band or song. Or maybe they did, and that's why people are having such a hard time with this new truth. But finally, after years of getting away with it, Weinstein is getting his just desserts while he's still alive; Johnny made it all the way to his grave without having to experience the shame and embarrassment of facing justice.
Maybe a bit of it comes from a sense of guilt by association; if we accept that the guy we liked before we found out he is/was actually a terrible person, that somehow indicts us... People have a really hard time separating their appreciation for an artist's work from their personal life. For most people who had no idea what was going on, it's just a matter of ignorance; for those who willingly ignored or buried stories in order to protect a monster, they're complicit in his crimes and there's still a chance for someone to face the music.