My guess is that, while the games technically run on chrome, they require the asm.js optimizations that only Firefox has atm in order to run at a decent speed.
I just played the demos in Chrome on the website and they run really well, except Dustforce which has a warning on it about only working well in Firefox.
1. https://blog.mozilla.org/futurereleases/2013/11/26/chrome-an...