Because npm 5 was a pretty big update that fundamentally changed how a lot of npm portions work, in order to close the gap between it and Yarn, a competing package manager. The 5.0 release was super rushed IMO (I personally hit several bugs), but if you stuck to node LTS you skipped over most of the v5 breakage (now it works 99.99% of the time again, and when it doesn't work deleting node_modules usually does the trick)