Because the underlying problems that pushed people toward Gamergate in the first place are s till there.
Millions of socially maladjusted mostly boys and men, eager to pin the blame for their social failures on anyone else rather than take a look at themselves and grow as a person -- because they have nobody to force them to understand that personally, or are otherwise so far gone that they wouldn't listen to anyone else anyway.
Gamergate works in the same way Nazi Party recruitment worked. They echo the problems people have, especially when those people don't understand the problem. They then point fingers at the groups they want to lambast, primarily women, PoC, LGBT, etc. in the gaming industry. They use examples for why they're bad, like, "It's censorship!" when talking about localization changes from Japan to the US market, or art changes, "They censor tiddies because they want to have a monopoly on them!" or they'll use whatever is convenient at the time, like Price, and openly defend MO even though multiple people have called him out explicitly for his leadership failures that apparently lead to harassment and toxic environment for women at Arenanet. "She harassed this guy for no reason, women like her have no place!"
Before, they were careful. They couldn't do it OPENLY for risk of exposing themselves and getting banned. But, for the most part, Youtube, Twitter, and Reddit didn't fucking ban them when they had the chance. An awful lot of people in Gamergate thought "ethics in games journalism" was the thing they were campaigning about, because when the movement was attacked, they would be on the front line. "I only care about ethics in games journalism!" These people were recruited with Driv3r and the Kane and Lynch firings, and once they were on board with that, the fake bullshit about Zoe Quinn linked those with a a woman. From there, they're that much more open to the use of stereotypes as dog whistles; portraying "gamer girls" as complete morons who "Don't even play games, so why would they want them changed?" (They tell you the reason -- it's because they hate YOU specifically.)
At the ground floor, "ethics in games journalism" seems like a reasonable take when you consider reviewers being fired because they didn't give a bad game good reviews, or publishers paying for good reviews of their games. But the 'leaders' of the gamergate movement knew that, and always intended to use them as a motte and bailey argument, while then turning around and accusing the other side of exactly what they were doing.
So to recap, maladjusted men who feel like they have something to lose because it's the only thing they have railing against a changing industry out of fear for what it would look like if it did change. Their numbers have declined on paper. They no longer have the crowds of people who talk about "Ethics in games journalism" because as time went on, it became very, very obvious that they were being used as a shield so others could just harass women and try to enforce a conservative worldview that, conveniently, had no place for women, PoC, or LGBT protagonists. Or antagonists. Or top billing, or anywhere at all.
But in practice, they're just as big as they ever were, just no longer part of the same movement.Instead, it's well-poisoning from individual people, communities dealing with perceived problems.
On the guildwars2 reddit right now, there's a post with the top comment being "See? It's JUST SOME women and outsiders and games journalists saying this is a problem, it's not a problem! See, look at this curated video and tell me it's a problem!" and "Jessica Price IS a story writer, she just wrote another story is all."
Instead of a single movement, it's now a movement across the entire industry centered in every single community, and the backlash from taking a good stand on the issue would rend these communities apart...because in practice, the number of maladjusted men has increased, the culture war, for now, appears to be leaning in their direction, they feel they can be open and honest about what they feel, and the moderation teams in these communities either sympathize with those ideas or fear the community won't be able to take it. Cowards all.
IT's still prevalent because it's still a feeling of community, it's still something people can identify with, and as it's an identity that gets rightfully attacked, it's something that fosters still greater sense of community. They should be banned on sight without a reason given.