Because often it’s not the team that chooses, but tooling is instead standardized across the enterprise. And enterprises like to make the “safe” choice of choosing what’s most popular. And then there’s the whole aspect that you have to know some basic Git anyway to debug your way through the open source code you use (maybe not for JavaScript/NPM, I don’t know). Git also happens to currently be the most interoperable with other kinds of tooling, from CI to IDEs, so not using Git makes your life harder in ways unrelated to its inherent qualities. It’s a network effect in multiple dimensions.

Git might be the best thing there is today (outside of very large companies or environments with large binaries). It doesn't mean that'll always be the case... There were other VCSs before git, and there will be after.