My first guess would be "Open hacker news" requires clear audible speech. While the KB method just requires pressing 'h' and 'enter'. Also, non-cloud speech recognition just recently got decent.
Context matters with such shortcuts. Even if I make sure that I have the location bar selected, focused and clear, 'h enter' takes me to a completely different website - because hacker news is actually 'news.ycombinator.com', so it's not the default by typing'ctrl-k h enter'.
And let's be frank. Even if we do get voice control, it's not going to somehow take our keyboards and mice away from us.
It also ought to be possible to specifically design interleaved voice+keyboard, voice+mouse, voice+touch, voice+pen, etc. interactions that could be more expressive and efficient than either input method by itself.