Generally nginx is written in C which simply performs better than garbage collected, dynamically typed javascript.

But regarding sending static files, nginx uses Linux' sendFile[0] which outperforms anything in user-space.

Up until a few years ago this wasn't something Node could use. I'm not sure if still that's the case though. My information could be badly outdated.

[0] http://man7.org/linux/man-pages/man2/sendfile.2.html