NaCl never gained cross-browser support. Mozilla called it "the antithesis of web standards" and refused to implement it. Apple ignored it. Microsoft backed TypeScript and asm.js instead. Developers do not want to write a plugin that runs on only 50% of the web (and later, just ~60% of desktop users).
: Unlike early versions of JavaScript, NaCl provided robust support for multi-threading, which is critical for complex software. Limitations & Current Status Browser Support : Support was almost exclusively limited to Google Chrome naclwebplugin
By 2015, it became clear that naclwebplugin was a dead end. Here is why: NaCl never gained cross-browser support