Media organizations like the New York Times have had unions for a long time. The Times Tech Guild is part of the New York Times Guild, which is part of the NewsGuild of New York, an umbrella organization for a lot of media unions in New York.
There is no tradition of unionization in most tech companies, and tech employees are paid very well and have usually had an easy time moving between companies. If you're unhappy with something, you can probably solve that as an individual without needing collective bargaining.
Tech workers at a company that's already unionized would be more likely to unionize in part because their colleagues are unionized, so they look around and say, "Hey, how come I'm not a part of that?" And the unions themselves can evangelize unions and recruit tech workers to unionize, which is good for the union because it gives them more resources and more bargaining partner. It's much harder for a union to come into a non-unionized workplace and start a movement from scratch, especially with a bunch of people who make six figures.
There's also a libertarian streak to Silicon Valley and the tech industry more broadly. This makes startup culture vibrant but at the expense of more individualism and less collectivism.
It's no coincidence that one of the few areas of tech to have seen a meaningful unionization push in recent years is gaming. Workers in gaming are in much more volatile positions, since it's a hit-driven business with long, expensive development cycles. And there's a constant stream of young, idealistic people who have dreamed of working in games their whole life and are willing to take on terrible working conditions and low pay to live that dream, at least for a while. There's also a lot of roles like art, music, and game design that are hard to parlay into other industries, whereas a software engineer or product manager who works can move between industries with relative ease. So there's an incentive for people in those roles to fix the companies and industries where they are instead of just moving on.