This is interesting. I wouldn't have assumed Metalsmith would be so far ahead for JS static site generators. Honestly, I've used a number of these (Metalsmith, Harp, Hexo and Wintersmith) and not been overly fond of any of them. Still much prefer Jekyll or Hugo - even as dev who favors JS.
Definitely take a look at Gatsby as well (it's react-based), I messed up the initial submission link and didn't use the right package name for Gatsby. (put in gatsby-cli instead of just gatsby)
https://npmcharts.com/compare/gatsby,metalsmith
The download count is more bursty for some reason
Ack, crap
Should be using just "gatsby" (which has a lot more downloads) than "gatsby-cli"
Here's the updated link:
https://npmcharts.com/compare/metalsmith,wintersmith,assemble,harp,brunch,nuxt,gatsby,hexo-fs,spike