It coming standard means one fewer dependency for everyone on the team to install, which is important on my current team where roughly half are backend- or mobile-only.
I like both yarn and npm, and yarn would have some benefits for us, but probably not enough to counter the extra effort onboarding other devs. npm still has some pain points, but I’ve ran into a few pain points on yarn too. And I think there being multiple projects here has helped the ecosystem.
npm is also doing some neat feature development (npm audit).
Also, the npm team is fantastic, whereas when I adopted yarn early on they dismissed the issue I opened about yarn not working for my setup. They’re a good team, but that lowered my confidence that I’d be able to get through any obstacles while using it. It’s still a fantastic tool though.)